selftests/vm: rename file run_vmtests to run_vmtests.sh

Commit c2aa8afc has renamed run_vmtests in Makefile, but the file
still uses the old name.

The kernel test robot reported the following issue:

  # selftests: vm: run_vmtests.sh
  # Warning: file run_vmtests.sh is missing!
  not ok 1 selftests: vm: run_vmtests.sh

Link: https://lkml.kernel.org/r/20210205085507.1479894-1-rong.a.chen@intel.com
Fixes: c2aa8afc (selftests/vm: rename run_vmtests --> run_vmtests.sh)
Signed-off-by: Rong Chen <rong.a.chen@intel.com>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>