mm, hugepages: make memory size variable in hugepage-mremap selftest

The hugetlb vma mremap() test currently maps 1GB of memory to trigger
pmd sharing and make sure that 'unshare' path in mremap code works.  The
test originally only mapped 10MB of memory (as specified by the header
comment) but was later modified to 1GB to tackle this case.

However, not all machines will have 1GB of memory to spare for this
test.  Adding a mapping size arg will allow run_vmtest.sh to pass an
adequate mapping size, while allowing users to run the test
independently with arbitrary size mappings.

Link: https://lkml.kernel.org/r/20211124203805.3700355-1-yosryahmed@google.com


Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Mina Almasry <almasrymina@google.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>