bcachefs: alloc_data_type_set() happens in alloc trigger

bcachefs: alloc_data_type_set() happens in alloc trigger

Originally, we ran insert triggers before overwrite so that if an extent
was being moved (by fallocate insert/collapse range), the bucket sector
count wouldn't hit 0 partway through, and so we don't trigger state
changes caused by that too soon.

But this is better solved by just moving the data type change to the
alloc trigger itself, where it's already called.

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