Fix #8661: fcntl(fd, F_GETLK, ...) violates POSIX

The standard states that F_GETLK should check whether given lock would be
blocked by another one and return description of the conflicting one (or
set l_type to F_UNLCK if there is no collision).

Current implementation of F_GETLK performs completely different actions, it
"Retrieves the first lock that has been set by the current team". Moreover,
if there are no locks (advisory_locking == NULL) an error is returned
instead of l_type set to F_UNLCK.
nfs4: VFS uses signed offsets while NFS4 expects unsigned