ext4: fix buffer leak in ext4_xattr_get_block() on error path

Fixes: dec214d0

 ("ext4: xattr inode deduplication")
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org # 4.13