utils: add helper functions useful for allocating a ring buffer

utils: add helper functions useful for allocating a ring buffer

This creates a mapping with twice the size of the allocated memory. The
second half of that mapping points at the same memory as the first half.

This is useful for ring buffers, because any read starting in the first
half can overflow into the second half as long as the read size is
smaller than the size of the memory area.

Signed-off-by: Felix Fietkau <nbd@nbd.name>