cachefiles: Preset the state xattr when creating a new file

With a really small cache, cachefiles is likely to see a lot of writes
hitting ENOSPC - and this can include setxattr that sets the state xattr on
a cachefile - but we don't really want to successfully fill a cache file
only to have to scrap it because we can't set the xattr.

Instead, preset the xattr when we create the tmpfile we're going to use,
and scrap the file at that point if we get ENOSPC.  Only if setxattr
succeeds do we allow data to be written to the file.

Note that there is a potential performance loss in that writes to the cache
have to be delayed until this is completed - but we do the tmpfile/setxattr
in parallel, starting when the file is opened and only have to wait once
writeback occurs.

Further, a new tracepoint, cachefiles_no_space, has been added to log ENOSPC
events, along with a couple of subreq traces to log failure detection in
netfslib.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Paulo Alcantara <pc@manguebit.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org