f2fs: avoid to drop nat entries due to the negative nr_shrink

The try_to_free_nats should not receive the negative nr_shrink.
Otherwise, it can drop all the nat entries by the while loop.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
f2fs: avoid unneeded lookup when xattr name length is too long

In f2fs_setxattr we have limit this attribute name length, so we should also
check it in f2fs_getxattr to avoid useless lookup caused by invalid name length.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>