fs: add basic sanity check before accessing the files array
This patch adds some basic file descriptor sanity checks to the file access
routines. Check whether the given file descriptor is in the files array range,
and whether the file entry is valid.
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>