dm: replace -EEXIST with -EBUSY

dm: replace -EEXIST with -EBUSY

The -EEXIST error code is reserved by the module loading infrastructure
to indicate that a module is already loaded. When a module's init
function returns -EEXIST, userspace tools like kmod interpret this as
"module already loaded" and treat the operation as successful, returning
0 to the user even though the module initialization actually failed.

This follows the precedent set by commit 54416fd76770 ("netfilter:
conntrack: helper: Replace -EEXIST by -EBUSY") which fixed the same
issue in nf_conntrack_helper_register().

Affected modules:
  * dm_cache dm_clone dm_integrity dm_mirror dm_multipath dm_pcache
  * dm_vdo dm-ps-round-robin dm_historical_service_time dm_io_affinity
  * dm_queue_length dm_service_time dm_snapshot

Signed-off-by: Daniel Gomez <da.gomez@samsung.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>