Fix remounting snapshots read-write

It's not enough to preserve/restore MS_RDONLY on the superblock flags
to avoid remounting a snapshot read-write: be explicit about our
intentions to the VFS layer so the readonly bit is updated correctly
in do_remount_sb().

Reviewed-by: Chunwei Chen <tuxoko@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #6510
Closes #6515