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
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