9p, afs, ceph, nfs: Use current_is_kswapd() rather than gfpflags_allow_blocking()

In 9p, afs ceph, and nfs, gfpflags_allow_blocking() (which wraps a
test for __GFP_DIRECT_RECLAIM being set) is used to determine if
->releasepage() should wait for the completion of a DIO write to fscache
with something like:

	if (folio_test_fscache(folio)) {
		if (!gfpflags_allow_blocking(gfp) || !(gfp & __GFP_FS))
			return false;
		folio_wait_fscache(folio);
	}

Instead, current_is_kswapd() should be used instead.

Note that this is based on a patch originally by Zhaoyang Huang[1].  In
addition to extending it to the other network filesystems and putting it on
top of my fscache rewrite, it also needs to include linux/swap.h in a bunch
of places.  Can current_is_kswapd() be moved to linux/mm.h?

Changes
=======
ver #5:
 - Dropping the changes for cifs.

Originally-signed-off-by: Zhaoyang Huang <[email protected]>
Co-developed-by: David Howells <[email protected]>
Signed-off-by: David Howells <[email protected]>
Reviewed-by: Jeff Layton <[email protected]>
cc: Zhaoyang Huang <[email protected]>
cc: Dominique Martinet <[email protected]>
cc: Marc Dionne <[email protected]>
cc: Steve French <[email protected]>
cc: Trond Myklebust <[email protected]>
cc: [email protected]
cc: [email protected]
cc: [email protected]
cc: [email protected]
cc: [email protected]
cc: [email protected]
cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]/ [1]
Link: https://lore.kernel.org/r/164021590773.640689.16777975200823659231.stgit@warthog.procyon.org.uk/ # v4

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