Avoid a null pointer dereference in zfs_mount() on FreeBSD

When mounting the root filesystem, vfs_t->mnt_vnodecovered is null

This will cause zfsctl_is_node() to dereference a null pointer when
mounting, or updating the mount flags, on the root filesystem, both
of which happen during the boot process.

Reported-by: Martin Matuska <mm@FreeBSD.org>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Allan Jude <allan@klarasystems.com>
Closes openzfs#14218

module/os/freebsd/zfs/zfs_vfsops.c

@@ -1328,7 +1328,8 @@ zfs_mount(vfs_t *vfsp)
 	}
 
 	fetch_osname_options(osname, &checkpointrewind);
-	isctlsnap = (zfsctl_is_node(mvp) && strchr(osname, '@') != NULL);
+	isctlsnap = (mvp != NULL && zfsctl_is_node(mvp) &&
+	    strchr(osname, '@') != NULL);
 
 	/*
 	 * Check for mount privilege?