crush/crush: ensure alignof(crush_work_bucket) is 1

crush/crush: ensure alignof(crush_work_bucket) is 1

in do_rule(), we allocate the space for crush_work_bucket using

char work[crush_work_size(crush, maxout)];

where crush_work_size() calculate the size like:

map->working_size + result_max * 3 * sizeof(__u32);

so work is allocated on stack, but the alignment of the
crush_work_bucket struct is not taken into consideration, so in
crush_init_workspace(), point could point to an address which is not
aligned to 8 bytes, which is the alignment of crush_work_bucket by
default. so is its member variables, all of them are uint32_t, and hence
are also 8-bytes aligned.

to ensure the compiler generate the correct assembly for accessing
the member variables without assuming that the struct is 8-byte
aligned, we should specify the alignment explicitly.

in this change, `__attribute__ ((packed))` is specified for
crush_work_bucket, so that its alignment is 1.

this issue is spotted by ASan, it complains like:

../src/crush/mapper.c:881:22: runtime error: member access within misaligned address 0x7ffe051f90dc for type 'struct crush_work_bucket', which requires 8 byte alignment
0x7ffe051f90dc: note: pointer points here
  1d e5 77 3d 68 55 00 00  00 00 00 00 00 00 00 00  20 93 1f 05 fe 7f 00 00  10 91 1f 05 fe 7f 00 00
              ^
../src/crush/mapper.c:882:22: runtime error: member access within misaligned address 0x7ffe051f90dc for type 'struct crush_work_bucket', which requires 8 byte alignment
0x7ffe051f90dc: note: pointer points here
  1d e5 77 3d 00 00 00 00  00 00 00 00 00 00 00 00  20 93 1f 05 fe 7f 00 00  10 91 1f 05 fe 7f 00 00
              ^
../src/crush/mapper.c:883:20: runtime error: member access within misaligned address 0x7ffe051f90dc for type 'struct crush_work_bucket', which requires 8 byte alignment
0x7ffe051f90dc: note: pointer points here
  1d e5 77 3d 00 00 00 00  00 00 00 00 00 00 00 00  20 93 1f 05 fe 7f 00 00  10 91 1f 05 fe 7f 00 00
              ^

Fixes: https://tracker.ceph.com/issues/50978
Signed-off-by: Kefu Chai <kchai@redhat.com>