VOP_CLOSE(): MNTK_EXTENDED_SHARED filesystems do not need excl lock

All in-tree implementations of VOP_CLOSE() for filesystems proclaiming
MNTK_EXTENDED_SHARED, are fine with the shared lock for the closed
vnode.  I checked the following implementations:
	ffs
	ext2
	ufs
	null
	tmpfs
	devfs
	fdescfs
	cd9660
	zfs
It seems that initial addition of FWRITE check was due to necessity of
handling the VV_TEXT vnode vflag.  Since VOP_ADD_WRITECOUNT() only
requires shared lock, we can relax the locking requirement there.

Reviewed by:	markj, Olivier Certner <olce.freebsd@certner.fr>
Tested by:	Olivier Certner
Sponsored by:	The FreeBSD Foundation
Differential revision:	https://reviews.freebsd.org/D39784

sys/kern/vfs_vnops.c

@@ -483,7 +483,7 @@ vn_open_vnode(struct vnode *vp, int fmode, struct ucred *cred,
 			 * If there is no fp, due to kernel-mode open,
 			 * we can call VOP_CLOSE() now.
 			 */
-			if ((vp->v_type == VFIFO || (fmode & FWRITE) != 0 ||
+			if ((vp->v_type == VFIFO ||
 			    !MNT_EXTENDED_SHARED(vp->v_mount)) &&
 			    VOP_ISLOCKED(vp) != LK_EXCLUSIVE)
 				vn_lock(vp, LK_UPGRADE | LK_RETRY);
@@ -528,11 +528,8 @@ vn_close1(struct vnode *vp, int flags, struct ucred *file_cred,
 	struct mount *mp;
 	int error, lock_flags;
 
-	if (vp->v_type != VFIFO && (flags & FWRITE) == 0 &&
-	    MNT_EXTENDED_SHARED(vp->v_mount))
-		lock_flags = LK_SHARED;
-	else
-		lock_flags = LK_EXCLUSIVE;
+	lock_flags = vp->v_type != VFIFO && MNT_EXTENDED_SHARED(vp->v_mount) ?
+	    LK_SHARED : LK_EXCLUSIVE;
 
 	vn_start_write(vp, &mp, V_WAIT);
 	vn_lock(vp, lock_flags | LK_RETRY);