selftests/mm: mremap_test: optimize execution time from minutes to seconds using chunkwise memcmp

Mismatch index is currently being checked by a brute force iteration over
the buffer.  Instead, break the comparison into O(sqrt(n)) number of
chunks, with the chunk size of this order only, where n is the size of the
buffer.  Do a brute-force iteration to print to stdout only when the
highly optimized memcmp() library function returns a mismatch in the
chunk.  The time complexity of this algorithm is O(sqrt(n)) * t, where t
is the time taken by memcmp(); for our test conditions, it is safe to
assume t to be small.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Dev Jain <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Kalesh Singh <[email protected]>
Cc: Shuah Khan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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