fs/fat: Fix pathnames using '..' that lead to the root directory

If we end up back in the root directory via a '..' directory entry, set
itr->is_root accordingly. Failing to do that gives spews like
"Invalid FAT entry" and being unable to access directory entries located
past the first cluster of the root directory.

Fixes: 8eafae20 ("fat/fs: convert to directory iterators")
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>