lib/memalign: don't malloc size twice

The footer offset (diff between malloc'd address and aligned address)
is (alignment-1) at most, and footer appears `size' bytes after the
aligned address, thus required size is (alignment-1 + size + sizeof(*f)).

The existing code seems to allocate extra unused `size' bytes.

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>