commit 18db4b4e upstream.

If some metadata block, such as an allocation bitmap, overlaps the
superblock, it's very likely that if the file system is mounted
read/write, the results will not be pretty.  So disallow r/w mounts
for file systems corrupted in this particular way.

Backport notes:
3.18.y is missing bc98a42c ("VFS: Convert sb->s_flags & MS_RDONLY to sb_rdonly(sb)")
and e462ec50 ("VFS: Differentiate mount flags (MS_*) from internal superblock flags")
so we simply use the sb MS_RDONLY check from pre bc98a42c in place of the sb_rdonly
function used in the upstream variant of the patch.
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org
Signed-off-by: Harsh Shandilya <harsh@prjkt.io>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>