bcachefs: Refactor bch2_bset_fix_lookup_table

bch2_bset_fix_lookup_table is too complicated to be easily understood,
the comment "l now > where" there is also incorrect when where ==
t->end_offset. This patch therefore refactor the function, the idea is
that when where >= rw_aux_tree(b, t)[t->size - 1].offset, we don't need
to adjust the rw aux tree.

Signed-off-by: Alan Huang <mmpgouride@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>