Btrfs: cleanup xattr code

Andrew's review of the xattr code revealed some minor issues that this patch
addresses.  Just an error return fix, got rid of a useless statement and
commented one of the trickier parts of __btrfs_getxattr.

Signed-off-by: Josef Bacik <jbacik@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>