dm-flakey: make corrupting read bios work

dm-flakey: make corrupting read bios work

dm-flakey corrupts the read bios in the endio function.  However, the
corrupt_bio_* functions checked bio_has_data() to see if there was data
to corrupt. Since this was the endio function, there was no data left to
complete, so bio_has_data() was always false. Fix this by saving a copy
of the bio's bi_iter in flakey_map(), and using this to initialize the
iter for corrupting the read bios. This patch also skips cloning the bio
for write bios with no data.

Reported-by: Kent Overstreet <kent.overstreet@linux.dev>
Fixes: a3998799fb4df ("dm flakey: add corrupt_bio_byte feature")
Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>