afs: Use new netfs lib read helper API

Make AFS use the new netfs read helpers to implement the VM read
operations:

 - afs_readpage() now hands off responsibility to netfs_readpage().

 - afs_readpages() is gone and replaced with afs_readahead().

 - afs_readahead() just hands off responsibility to netfs_readahead().

These make use of the cache if a cookie is supplied, otherwise just call
the ->issue_op() method a sufficient number of times to complete the entire
request.

Changes:
v5:
- Use proper wait function for PG_fscache in afs_page_mkwrite()[1].
- Use killable wait for PG_writeback in afs_page_mkwrite()[1].

v4:
- Folded in error handling fixes to afs_req_issue_op().
- Added flag to netfs_subreq_terminated() to indicate that the caller may
  have been running async and stuff that might sleep needs punting to a
  workqueue.

Signed-off-by: David Howells <[email protected]>
Tested-By: Marc Dionne <[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/160588542733.3465195.7526541422073350302.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/161118158436.1232039.3884845981224091996.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/161161053540.2537118.14904446369309535330.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/161340418739.1303470.5908092911600241280.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/161539561926.286939.5729036262354802339.stgit@warthog.procyon.org.uk/ # v4
Link: https://lore.kernel.org/r/161653817977.2770958.17696456811587237197.stgit@warthog.procyon.org.uk/ # v5
Link: https://lore.kernel.org/r/161789101258.6155.3879271028895121537.stgit@warthog.procyon.org.uk/ # v6

Author: dhowells

fs/afs/Kconfig

@@ -4,6 +4,7 @@ config AFS_FS
 	depends on INET
 	select AF_RXRPC
 	select DNS_RESOLVER
+	select NETFS_SUPPORT
 	help
 	  If you say Y here, you will get an experimental Andrew File System
 	  driver. It currently only supports unsecured read-only AFS access.