EX-7601 tests: add mmap write test

EX-7601 tests: add mmap write test

This improves the existing mmap test to test mmap writing
as well as mmap reading.

Test-Parameters: trivial
Test-Parameters: testlist=sanity-compr env=ONLY="1003",ONLY_REPEAT=10
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I81840c7bbbefbc5c3bae6b270c2d94297a254d19
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53307
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>