llist: add llist_del_first_this()

llist_del_first_this() deletes a specific entry from an llist, providing
it is at the head of the list.  Multiple threads can call this
concurrently providing they each offer a different entry.

This can be uses for a set of worker threads which are on the llist when
they are idle.  The head can always be woken, and when it is woken it
can remove itself, and possibly wake the next if there is an excess of
work to do.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>