This introduces some new conveniences, to help cut down on boilerplate:

 - bch2_trans_kmalloc_nomemzero() - performance optimiation
 - bch2_bkey_make_mut()
 - bch2_bkey_get_mut()
 - bch2_bkey_get_mut_typed()
 - bch2_bkey_alloc()
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>