fs: dentry cache implementation ...

This adds the Linux dentry cache implementation to barebox.

Until now every filesystem driver resolves the full path to a file for
itself. This leads to code duplication and is error prone since
resolving paths is a complicated task. Also it can narrow down the
lookup performance since barebox only knows ASCII paths and has no way
of caching lookups.

With this patch we get the Linux dcache implementation. The path
resolving code from fs/namei.c is nearly taken as-is, minus the RCU
and locking code. Dcaching is made simple as of now: We simply cache
everything and never release any dentries. Although we do reference
counting for inodes and dentries it is effectively not used yet.
We never free anything until a fs is unmounted in which case we free
everything no matter if references are taken or not.

This patch also contains a wrapper in fs/legacy.c to support
filesystems with the old API.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

squashfs: Add zstd support ...

Based on kernel commit 87bf54bb43dd ("squashfs: Add zstd support").

Creating zstd compressed squashfs images is supported by squashfs-tools
newer than commit e38956b92f73 ("squashfs-tools: Add zstd support").

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

fs: squashfs: Enable squashfs XZ support when XZ is enabled ...

XZ support itself is much more expensive than the squashfs XZ wrapper,
so it makes sense to just compile XZ support into barebox when XZ
support is available. No need to ask the user for it.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

fs: squashfs: port zlib compression support from kernel ...

As this is the default compression method for squashfs, make this the
default in kconfig selection, too

Signed-off-by: Enrico Jorns <ejo@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

fs: squashfs: port lz4 compression support from kernel ...

Signed-off-by: Enrico Jorns <ejo@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

fs: squashfs: port lzo compression support from kernel ...

This ports lzo_wrapper from kernel code and adds some minimal adaptions
to make squashfs lzo compression work in barebox.

Signed-off-by: Enrico Jorns <ejo@pengutronix.de>
Tested-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

fs: add support for SquashFS 4.0 ...

The driver was imported from Linux 4.4.

Current implementation supports only XZ decompressor.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Tested-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>