xfs: check for cow blocks before trying to clear them

There's no point in allocating a transaction and locking the inode in
preparation to clear cow blocks if there actually are any cow fork
extents.  Therefore, move the xfs_reflink_cancel_cow_range hunk to
xfs_inactive and check the cow ifp first.  This makes inode reclamation
run faster.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>