memfd test should be memfd_create test

memfd test should be memfd_create test

Currently, syscall memfd_create was tested by this:

$ perl tests/vg_regtest memcheck/tests/linux/memfd.vgtest

Since memfd_secret introduced in kernel 5.14, valgrind should rename
the "memfd" test to "memfd_create" test to avoid the ambiguity, so that
user will not get confused with the "memfd_secret" test.

After this change, syscall memfd_create will be tested by:

$ perl tests/vg_regtest memcheck/tests/linux/memfd_create.vgtest