netfs: Pass more information on how to deal with a hole in the cache

Pass more information to the cache on how to deal with a hole if it
encounters one when trying to read from the cache.  Three options are
provided:

 (1) NETFS_READ_HOLE_IGNORE.  Read the hole along with the data, assuming
     it to be a punched-out extent by the backing filesystem.

 (2) NETFS_READ_HOLE_CLEAR.  If there's a hole, erase the requested region
     of the cache and clear the read buffer.

 (3) NETFS_READ_HOLE_FAIL.  Fail the read if a hole is detected.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819612321.215744.9738308885948264476.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906914460.143852.6284247083607910189.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967119923.1823006.15637375885194297582.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021519762.640689.16994364383313159319.stgit@warthog.procyon.org.uk/ # v4