selftests/vm/.gitignore: add mremap_dontunmap

Add mremap_dontunmap to .gitignore.

Fixes: 0c28759e ("selftests: add MREMAP_DONTUNMAP selftest")
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Brian Geffon <bgeffon@google.com>
Link: http://lkml.kernel.org/r/20200517002509.362401-2-jhubbard@nvidia.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/vm/.gitignore

@@ -6,6 +6,7 @@ map_populate
 thuge-gen
 compaction_test
 mlock2-tests
+mremap_dontunmap
 on-fault-limit
 transhuge-stress
 userfaultfd