netfs: Record contention stats for writeback lock

Record statistics for contention upon the writeback serialisation lock that
prevents racing writeback calls from causing each other to interleave their
writebacks.  These can be viewed in /proc/fs/netfs/stats on the WbLock line,
with skip=N indicating the number of non-SYNC writebacks skipped and wait=N
indicating the number of SYNC writebacks that waited.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: Steve French <sfrench@samba.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/20240814203850.2240469-5-dhowells@redhat.com/

 # v2
Signed-off-by: Christian Brauner <brauner@kernel.org>