xfs: fold xfs_attr_get_int into xfs_attr_get

This allows doing an unlocked check if an attr for is present at all and
slightly reduce the lock hold time if we actually do an attr get.

Plus various minor style fixes to the new xfs_attr_get.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>