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.

Reviewed by: alc, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D22874