lightnvm: propagate pblk_init return to userspace

lightnvm: propagate pblk_init return to userspace

From userspace calling ioctl(NVM_DEV_CREATE) was returning ENOMEM for
invalid arguments even though pblk (pblk_init) was returning correctly
-EINVAL to nvm_create_tgt inside core.  This patch propagates the
correct return value to userspace.

Because pblk was introduced recently this only needs to go in 4.12.

Fixes: a4bd217b4326 ("lightnvm: physical block device (pblk) target")
Signed-off-by: Rakesh Pandit <rakesh@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>