xfs: drop ->writepage completely

->writepage is only used in one place - single page writeback from
memory reclaim. We only allow such writeback from kswapd, not from
direct memory reclaim, and so it is rarely used. When it comes from
kswapd, it is effectively random dirty page shoot-down, which is
horrible for IO patterns. We will already have background writeback
trying to clean all the dirty pages in memory as efficiently as
possible, so having kswapd interrupt our well formed IO stream only
slows things down. So get rid of xfs_vm_writepage() completely.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
[djwong: forward port to 5.15]
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>