In __canonicalize_path() we dereference the pointer returned by
get_fsdevice_by_path() without checking if the pointer is NULL or not.
When the pointer is NULL it leads to an Ooops.

	Ooops, address error on load or ifetch!

	$ 0   : 00000000 00000001 a0000026 a0811c10
	$ 4   : a0402e60 a0402e48 a0811c00 a0402e58
	$ 8   : 00000001 00000000 0000005a 00000023
	$12   : 00000000 00000002 00601021 00000000
	$16   : a0402e60 a0402e50 a0402e39 a0810000
	$20   : a0402e38 a0811420 a0811424 00000000
	$24   : 00000000 a080de10
	$28   : 87f87d40 a03ffa68 a0810000 a080ce30
	Hi    : 00000002
	Lo    : 00000000
	epc   : a080ce34
	ra    : a080ce30
	Status: 00000006
	Cause : 40008010
	Config: 80040483

	### ERROR ### Please RESET the board ###

Fixes: d79a81736 fs: Don't bother filesystems without link support with additional stat() calls

Signed-off-by: Gaël PORTAY <gael.portay@savoirfairelinux.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>