Make statfs properly return read-only state after emergency remount

Emergency remount (sysrq-u) sets MS_RDONLY to the superblock but doesn't set
MNT_READONLY to the mount point.

Once calculate_f_flags() only check for the mount point read only state,
when setting kstatfs flags, after an emergency remount, statfs does not
report the filesystem as read-only, even though it is.

Enable flags_by_sb() to also check for superblock read only state, so the
kstatfs and consequently statfs can properly show the read-only state of
the filesystem.
Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/statfs.c

@@ -38,6 +38,8 @@ static int flags_by_sb(int s_flags)
 		flags |= ST_SYNCHRONOUS;
 	if (s_flags & MS_MANDLOCK)
 		flags |= ST_MANDLOCK;
+	if (s_flags & MS_RDONLY)
+		flags |= ST_RDONLY;
 	return flags;
 }