rbd: use kmalloc_array() in rbd_header_from_disk()
* A multiplication for the size determination of a memory allocation
indicated that an array data structure should be processed.
Thus use the corresponding function "kmalloc_array".
This issue was detected by using the Coccinelle software.
* Delete the local variable "size" which became unnecessary with
this refactoring.
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net> Signed-off-by: Ilya Dryomov <idryomov@gmail.com>