bcachefs: Fix bch2_extent_fallocate() in nocow mode

bcachefs: Fix bch2_extent_fallocate() in nocow mode

When we allocate disk space, we need to be incrementing the WRITE io
clock, which perhaps should be renamed to sectors allocated - copygc
uses this io clock to know when to run.

Also, we should be incrementing the same clock when allocating btree
nodes.

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