unionfs_lock(): eliminate LK_CANRECURSE special-case

As of commit 9f5c4ef328 ("dounmount(9): temporarily enable recursion
for the covered vnode lock"), the unmount path handles recursion
automatically, so there's no longer a need to handle this case
in unionfs-specific code.

Reviewed by:	kib, markj
Tested by:	pho
Differential Revision:	https://reviews.freebsd.org/D58858

sys/fs/unionfs/union_vnops.c

@@ -2252,17 +2252,6 @@ unionfs_lock_restart:
 		lvp_locked = true;
 	}
 
-	/*
-	 * During unmount, the root vnode lock may be taken recursively,
-	 * because it may share the same v_vnlock field as the vnode covered by
-	 * the unionfs mount.  The covered vnode is locked across VFS_UNMOUNT(),
-	 * and the same lock may be taken recursively here during vflush()
-	 * issued by unionfs_unmount().
-	 */
-	if ((flags & LK_TYPE_MASK) == LK_EXCLUSIVE &&
-	    (vp->v_vflag & VV_ROOT) != 0)
-		flags |= LK_CANRECURSE;
-
 	vholdnz(tvp);
 	VI_UNLOCK(vp);
 	error = VOP_LOCK(tvp, flags);