block.c: cleanup

block.c: cleanup

* block.c: Include <inttypes.h>.
(print_blkpg_req): Always decode struct blkpg_ioctl_arg.
Robustify decoding of strings.
(block_ioctl): Do not decode return values passed by pointers on exit
from failed syscalls.
Use format macros from inttypes.h to print values of type uint64_t.