mm: honor FGP_NOWAIT for page cache page allocation

If we're creating a page cache page with FGP_CREAT but FGP_NOWAIT is
set, we should dial back the gfp flags to avoid frivolous blocking
which is trivial to hit in low memory conditions:

[   10.117661]  __schedule+0x8c/0x550
[   10.118305]  schedule+0x58/0xa0
[   10.118897]  schedule_timeout+0x30/0xdc
[   10.119610]  __wait_for_common+0x88/0x114
[   10.120348]  wait_for_completion+0x1c/0x24
[   10.121103]  __flush_work.isra.0+0x16c/0x19c
[   10.121896]  flush_work+0xc/0x14
[   10.122496]  __drain_all_pages+0x144/0x218
[   10.123267]  drain_all_pages+0x10/0x18
[   10.123941]  __alloc_pages+0x464/0x9e4
[   10.124633]  __folio_alloc+0x18/0x3c
[   10.125294]  __filemap_get_folio+0x17c/0x204
[   10.126084]  iomap_write_begin+0xf8/0x428
[   10.126829]  iomap_file_buffered_write+0x144/0x24c
[   10.127710]  xfs_file_buffered_write+0xe8/0x248
[   10.128553]  xfs_file_write_iter+0xa8/0x120
[   10.129324]  io_write+0x16c/0x38c
[   10.129940]  io_issue_sqe+0x70/0x1cc
[   10.130617]  io_queue_sqe+0x18/0xfc
[   10.131277]  io_submit_sqes+0x5d4/0x600
[   10.131946]  __arm64_sys_io_uring_enter+0x224/0x600
[   10.132752]  invoke_syscall.constprop.0+0x70/0xc0
[   10.133616]  do_el0_svc+0xd0/0x118
[   10.134238]  el0_svc+0x78/0xa0

Clear IO, FS, and reclaim flags and mark the allocation as GFP_NOWAIT and
add __GFP_NOWARN to avoid polluting dmesg with pointless allocations
failures. A caller with FGP_NOWAIT must be expected to handle the
resulting -EAGAIN return and retry from a suitable context without NOWAIT
set.
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

mm/filemap.c

@@ -1988,6 +1988,10 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
 			gfp |= __GFP_WRITE;
 		if (fgp_flags & FGP_NOFS)
 			gfp &= ~__GFP_FS;
+		if (fgp_flags & FGP_NOWAIT) {
+			gfp &= ~GFP_KERNEL;
+			gfp |= GFP_NOWAIT | __GFP_NOWARN;
+		}
 
 		folio = filemap_alloc_folio(gfp, 0);
 		if (!folio)