common/buffer: adjust align before calling posix_memalign()

common/buffer: adjust align before calling posix_memalign()

posix_memalign() requires alignment argument to be a multiple of
sizeof(void *).  Since it is an implementation detail of buffer,
it needs to be adjusted there -- buffer consumers have no way of
knowing that passing e.g. align == 4 is incorrect.

One place already does the adjustment, but only for align == 0.
The other just asserts.  Fix both and remove the "power of two"
assertion.  Let posix_memalign() return EINVAL and handle that
by throwing buffer::bad_alloc, as expected by the consumers.

Fixes: https://tracker.ceph.com/issues/50646
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>