bcachefs: BCH_SB_LAYOUT_SIZE_BITS_MAX

Define a constant for the max superblock size, to avoid a too-large
shift.

Reported-by:  <syzbot+a8b0fb419355c91dda7f@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>