Pass MNTK_NO_IOPF and MNTK_UNMAPPED_BUFS flags from the lower
filesystem to the nullfs mount.

MNTK_NO_IOPF must be present on the nullfs struct mount so that struct
file fo_read and fo_write fops operate in the mode requested by the
lower mount.

MNTK_UNMAPPED_BUFS allows VOP_GETPAGES() to use unmapped buffers. It
does not matter for VOP_GETPAGES() calls from vm_fault() since handle
of the vm_object always points to the lower vnode. But it may be
useful for other situations where VOP_GETPAGES() is used.

Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks