BCH_IOCTL_FSCK_OFFLINE allows the userspace fsck tool to use the kernel
implementation of fsck - primarily when the kernel version is a better
version match.

It should look and act exactly like the normal userspace fsck that the
user expected to be invoking, so errors should never result in a kernel
panic.

We may want to consider further restricting errors=panic - it's only
intended for debugging in controlled test environments, it should have
no purpose it normal usage.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>