memblock: split memblock_find_base() out of __memblock_alloc_base()

This will be used by the array resize code and might prove useful
to some arch code as well at which point it can be made non-static.

Also add comment as to why aligning size is important

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
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v2. Fix loss of size alignment
v3. Fix result code