mm: Clarify grub_real_malloc()

mm: Clarify grub_real_malloc()

When iterating through the singly linked list of free blocks,
grub_real_malloc() uses p and q for the current and previous blocks
respectively. This isn't super clear, so swap to using prev and cur.

This makes another quirk more obvious. The comment at the top of
grub_real_malloc() might lead you to believe that the function will
allocate from *first if there is space in that block.

It actually doesn't do that, and it can't do that with the current
data structures. If we used up all of *first, we would need to change
the ->next of the previous block to point to *first->next, but we
can't do that because it's a singly linked list and we don't have
access to *first's previous block.

What grub_real_malloc() actually does is set *first to the initial
previous block, and *first->next is the block we try to allocate
from. That allows us to keep all the data structures consistent.

Document that.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>