bcachefs: BCH_IOCTL_FSCK_OFFLINE

This adds a new ioctl for running fsck on a list of devices.

Normally, if we wish to use the kernel's implementation of fsck we'd run
it at mount time with -o fsck. This ioctl lets us run fsck without
mounting, so that userspace bcachefs-tools can transparently switch to
the kernel's implementation of fsck when appropriate - primarily if the
kernel version of bcachefs better matches the filesystem on disk.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>