bcachefs: Fix a kasan splat in bch2_dev_add()

This fixes a use after free - mi is dangling after the resize call.

Additionally, resizing the device's member info section was useless - we
were attempting to preallocate the space required before adding it to
the filesystem superblock, but there's other sections that we should
have been preallocating as well for that to work.

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