selftest/mremap_test: update the test to handle pagesize other than 4K

Patch series "mrermap fixes", v2.

This patch (of 6):

Instead of hardcoding 4K page size fetch it using sysconf().  For the
performance measurements test still assume 2M and 1G are hugepage sizes.

Link: https://lkml.kernel.org/r/20210616045239.370802-1-aneesh.kumar@linux.ibm.com
Link: https://lkml.kernel.org/r/20210616045239.370802-2-aneesh.kumar@linux.ibm.com
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Reviewed-by: Kalesh Singh <kaleshsingh@google.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>