backports: do not add debugfs_create_x64() on RHEL 6.4

Upstream commit: a00d7b07a89d1bb0b6c9a51bfec4e53a76940bb3

debugfs_create_x64() is provided by the RHEL 6.4 kernel and should not
be added to the build on such a kernel.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>