vnode: remove VLOCKSWORK and check locking when vop_islocked != nullop

vnode: remove VLOCKSWORK and check locking when vop_islocked != nullop

This flag is currently used to mark or unmark a vnode to actively
check vnode locking semantic (when compiled with VFSLCKDEBUG).

Currently, VLOCKSWORK flag isn't properly set for several FS
implementation which have full locking support. This commit enable
proper checking for them too (cd9660, udf, fuse, msdosfs, tmpfs).

Instead of using a particular flag, it directly check if
v_op->vop_islocked is nullop or not to activate or not the vnode
locking checks.

ok mpi@