netfs: Adjust the netfs_failure tracepoint to indicate non-subreq lines

Adjust the netfs_failure tracepoint to indicate a subrequest number of -1
when it's a full-request failure unrelated to any particular subrequest,
such as a failure to encrypt its data buffer.

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/164623001948.3564931.2353852999649380059.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/164678204587.1200972.14893513018190383961.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/164692903233.2099075.15414355147237641274.stgit@warthog.procyon.org.uk/ # v3