bcachefs: Improve can_write_extent()

This fixes another "rebalance spinning and doing no work" issue;
rebalance was reading extents it wanted to move, but then failing in
bch2_write() -> bch2_alloc_sectors_start() due to being unable to
allocate sufficient replicas.

This was triggered by a user playing with the durability settings, the
foreground device was an NVME device with durability=2, and originally
he'd set the background device to durability=2 as well, but changed it
back to 1 (the default) after seeing IO errors.

That meant that with replicas=2, we want to move data off the NVME
device which satisfies that constraint, but with a single durability=1
device on the background target there's no way to move the extent to
that target while satisfiying the "required replicas" constraint.

The solution for now is for bch2_data_update_init() to check for this,
and return an error - before kicking off the read.

bch2_data_update_init() already had two different checks for "will we be
able to write this extent", with partially duplicated code, so this
patch combines and improves that logic.

Additionally, we now always bail out and return an error if there's
insufficient space on the destination target. Previously, we only did
this for BCH_WRITE_alloc_nowait moves, because it might be the case that
copygc just needs to free up space on the destination target.

But we really shouldn't kick off a move if the destination is full, we
can't currently distinguish between "really full" and "just need to wait
for copygc", and if we are going to wait on copygc it'd be better to do
that before kicking off the move.

This will additionally fix "rebalance spinning" issues caused by a
filesystem that has more data than can fit in background_target - which
is a valid scenario, since we don't exclude foreground/cache devices
when calculating filesystem capacity.

Reported-by: Maël Kerbiriou <mael.kerbiriou@free.fr>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

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