bcachefs: Fix UAF in bchfs_read()

Commit 3ba0240a fixed a bug in the read retry path in __bch2_read(),
and changed bchfs_read() to match - to avoid a landmine if
bch2_read_extent() ever starts returning transaction restarts.

But that was incorrect, because bchfs_read() doesn't use a separate
stack allocated bvec_iter, it uses the one in the rbio being submitted.

Add a comment explaining the issue, and revert the buggy change.

Fixes: 3ba0240a ("bcachefs: Fix silent short reads in data read retry path")
Reported-by:  <syzbot+2deb10b8dc9aae6fab67@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>