xfs: rename xfs_iomap_swapfile_activate to xfs_vm_swap_activate

Match the method name and the naming convention or address_space
operations.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>