netfs: Prepare to split read_helper.c

Rename netfs_rreq_unlock() to netfs_rreq_unlock_folios() to make it sound
less like it's dropping a lock on an netfs_io_request struct.

Remove the 'static' marker on netfs_rreq_unlock_folios() and declaring it
in internal.h preparatory to splitting the file.

Changes
=======
ver #2)
 - Slide this patch to after the one adding netfs_begin_read().
 - As a consequence, don't need to unstatic so many functions.

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/164623002861.3564931.17340149482236413375.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/164678215208.1200972.9761906209395002182.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/164692912709.2099075.4349905992838317797.stgit@warthog.procyon.org.uk/ # v3