dm-crypt: enable DM_TARGET_ATOMIC_WRITES

dm-crypt: enable DM_TARGET_ATOMIC_WRITES

Allow handling of bios with REQ_ATOMIC flag set.

Don't split these bios and fail them if they overrun the hard limit
"BIO_MAX_VECS << PAGE_SHIFT".

In order to simplify the code, this commit joins the logic that avoids
splitting emulated zone append bios with the logic that avoids
splitting atomic write bios.

Signed-off-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Tested-by: John Garry <john.g.garry@oracle.com>