bcachefs: Add block plugging to read paths

bcachefs: Add block plugging to read paths

This will help with some of the btree_trans srcu lock hold time warnings
that are still turning up; submit_bio() can block for awhile if the
device is sufficiently congested.

It's not a perfect solution since blk_plug bios are submitted when
scheduling; we might want a way to disable the "submit on context
switch" behaviour, or switch to our own plugging in the future.

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