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.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>