f2fs: reorganize f2fs_setxattr

make use of F2FS_NAME_LEN for name length checking,
change return conditions at few places, by assigning
storing the errorvalue in 'error' and making a common
exit path.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Amit Sahrawat <a.sahrawat@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>