kernel - Fix recursive lockmgr lock panic when creating a hardlink
* HAMMER uses non-recursive vnode locks. Fix a case where attempting
to hardlink A to B where both already exist and point to the same
inode can result in a panic if B is not in the namecache.
HAMMER must temporarily lock the vnode it returns in the resolver
and kern_link() was already holding A locked. This all occurs
before kern_link() is able to check whether A and B are the same
so it can't be avoided.
Solve the problem by unlocking A while resolving B.
Reported-by: Goetz Isenmann <info@goetz-isenmann.de>