common/bloom_filter: return by return values not by input params

common/bloom_filter: return by return values not by input params

it'd be easier for the static analyzer (like GCC), to reason about if
a variable is initialized before being used.

this change also helps to improve the readability, and to silence the
false alarm like:

In file included from ../src/os/bluestore/BlueStore.h:42,
                 from ../src/os/bluestore/BlueStore.cc:26:
../src/common/bloom_filter.hpp: In member function 'void std::vector<_Tp, _Alloc>::_M_fill_insert(std::vector<_Tp, _Alloc>::iterator, std::vector<_Tp, _Alloc>::size_type, const value_type&) [with _Tp = bloom_filter; _Alloc = mempool::pool_allocator<mempool::mempool_bluestore_fsck, bloom_filter>]':
../src/common/bloom_filter.hpp:118:46: warning: '*((void*)(& __tmp)+8).bloom_filter::table_size_' may be used uninitialized in this function [-Wmaybe-uninitialized]
  118 |  mempool::bloom_filter::alloc_byte.deallocate(bit_table_, table_size_);
      |  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Kefu Chai <kchai@redhat.com>