bcachefs: More improvements for alloc info checks

bcachefs: More improvements for alloc info checks

 - Move checks for whether the device & bucket are valid from the
   .key_invalid method to bch2_check_alloc_key(). This is because
   .key_invalid() is called on keys that may no longer exist (post
   journal replay), which is a problem when removing/resizing devices.

 - We weren't checking the need_discard btree to ensure that every set
   bucket has a corresponding alloc key. This refactors the code for
   checking the freespace btree, so that it now checks both.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>