storage: avoid maybe-uninitialized warning by GCC 10

storage: avoid maybe-uninitialized warning by GCC 10

GCC 10 complains about variables may be used uninitialized.
Even though it might be false positives, we can easily avoid them.

Avoiding
 ../src/storage/storage_backend_iscsi_direct.c:634:11: error: ‘nb_block’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
   634 |     while (lba < nb_block) {
       |           ^
 ../src/storage/storage_backend_iscsi_direct.c:619:14: note: ‘nb_block’ was declared here
   619 |     uint64_t nb_block;
       |              ^~~~~~~~
 ../src/storage/storage_backend_iscsi_direct.c:637:16: error: ‘block_size’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
   637 |         task = iscsi_write16_sync(iscsi, lun, lba, data,
       |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   638 |                                   block_size * to_write,
       |                                   ~~~~~~~~~~~~~~~~~~~~~~
   639 |                                   block_size, 0, 0, 0, 0, 0);
       |                                   ~~~~~~~~~~~~~~~~~~~~~~~~~~
 ../src/storage/storage_backend_iscsi_direct.c:618:14: note: ‘block_size’ was declared here
   618 |     uint32_t block_size;
       |              ^~~~~~~~~~
 ../src/storage/storage_backend_iscsi_direct.c: In function ‘virStorageBackendISCSIDirectRefreshPool’:
 ../src/storage/storage_backend_iscsi_direct.c:320:39: error: ‘nb_block’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
   320 |     vol->target.capacity = block_size * nb_block;
       |                            ~~~~~~~~~~~^~~~~~~~~~
 ../src/storage/storage_backend_iscsi_direct.c:306:14: note: ‘nb_block’ was declared here
   306 |     uint64_t nb_block;
       |              ^~~~~~~~
 ../src/storage/storage_backend_iscsi_direct.c:320:39: error: ‘block_size’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
   320 |     vol->target.capacity = block_size * nb_block;
       |                            ~~~~~~~~~~~^~~~~~~~~~
 ../src/storage/storage_backend_iscsi_direct.c:305:14: note: ‘block_size’ was declared here
   305 |     uint32_t block_size;
       |              ^~~~~~~~~~

Signed-off-by: Boris Fiuczynski <fiuczy@linux.ibm.com>
Reviewed-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>