bcachefs: Improve handling of extents in bch2_trans_update()

bcachefs: Improve handling of extents in bch2_trans_update()

The transaction update/commit path cares about whether it's inserting
extents or regular keys; extents require extra passes (handling of
overlapping extents) but sometimes we want to skip all that. This
clarifies things by adding a new member to btree_insert_entry specifying
whether the key being inserted is an extent, instead of overloading
BTREE_ITER_IS_EXTENTS.

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