vfs: Undo an overly zealous MS_RDONLY -> SB_RDONLY conversion

vfs: Undo an overly zealous MS_RDONLY -> SB_RDONLY conversion

BugLink: http://bugs.launchpad.net/bugs/1794889
[ Upstream commit a9e5b73288cf1595ac2e05cf1acd1924ceea05fa ]

In do_mount() when the MS_* flags are being converted to MNT_* flags,
MS_RDONLY got accidentally convered to SB_RDONLY.

Undo this change.

Fixes: e462ec50cb5f ("VFS: Differentiate mount flags (MS_*) from internal superblock flags")
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>