bcachefs: make offline fsck set read_only fs flag

bcachefs: make offline fsck set read_only fs flag

A subsequent change will remove "read_only" as a mount option in favor
of the standard option "ro", meaning the userspace fsck command cannot
pass it to the fsck ioctl. Instead, in offline fsck, set "read_only"
kernel-side without trying to parse it as a mount option.

For compatibility with versions of the "bcachefs fsck" command that try
to pass the "read_only" mount opt, remove it from the mount options
string prior to parsing when it is present.

Signed-off-by: Thomas Bertschinger <tahbertschinger@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>