dm: fix dm_io and dm_target_io flags race condition on Alpha

Early alpha processors cannot write a single byte or short; they read 8
bytes, modify the value in registers and write back 8 bytes.

This could cause race condition in the structure dm_io - if the fields
flags and io_count are modified simultaneously.

Fix this bug by using 32-bit flags if we are on Alpha and if we are
compiling for a processor that doesn't have the byte-word-extension.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: bd4a6dd2

 ("dm: reduce size of dm_io and dm_target_io structs")
[snitzer: Jens allowed this change since Mikulas owns a relevant Alpha!]
Acked-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>