common: add bufferlist::rebuild_aligned_size_and_memory

common: add bufferlist::rebuild_aligned_size_and_memory

The function bufferlist::rebuild_aligned checks memory and size
alignment with the same variable. It is however useful to separate
memory alignment constraints from size alignment constraints. For
instance rebuild_aligned could be called to allocate an erasure coded
buffer where each 2048 bytes chunk needs to start on a memory address
aligned on 32 bytes.

Signed-off-by: Loic Dachary <loic@dachary.org>