Including <sys/tmpfs.h> into non-kernel software leads to a compilation error because, without _KERNEL defined, the macro TMPFS_VALIDATE_DIR is invoked, but never defined. User-level software that includes sys/tmpfs.h must define _KERNEL to make the definition of TMPFS_VALIDATE_DIR visible.
This change puts all the inline functions that, directly or indirectly, invoke MPASS into the scope of the _KERNEL block, allowing many user-space includers of <sys/tmpfs.h> to stop defining _KERNEL.