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Benchmark the time it takes for virtmem to handle pagefault with noop
loadblk for loadblk both implemented in C and in Python.

On my computer it is:

	name          µs/op
	PagefaultC    269 ± 0%
	pagefault_py  291 ± 0%

Quite a big time in other words.

It turned out to be mostly spent in fallocate'ing pages on tmpfs from
/dev/shm. Part of the above 269 µs/op is taken by freeing (reclaiming)
pages back when benchmarking work size exceed /dev/shm size, and part to
allocating.

If I limit the work size (via npage in benchmem.c) to be less than whole
/dev/shm it starts to be ~ 170 µs/op and with additional tracing it
shows as something like this:

    	.. on_pagefault_start   0.954 µs
    	.. vma_on_pagefault_pre 0.954 µs
    	.. ramh_alloc_page_pre  0.954 µs
    	.. ramh_alloc_page      169.992 µs
    	.. vma_on_pagefault     172.853 µs
    	.. vma_on_pagefault_pre 172.853 µs
    	.. vma_on_pagefault     174.046 µs
    	.. on_pagefault_end     174.046 ...