bcachefs: Fix silent short reads in data read retry path

bcachefs: Fix silent short reads in data read retry path

__bch2_read, before calling __bch2_read_extent(), sets bvec_iter.bi_size
to "the size we can read from the current extent" with a swap, and
restores it to "the size for the total read" after the read_extent call
with another swap.

But we neglected to do the restore before the "if (ret) goto err;" -
which is a problem if we're retrying those errors.

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