vdso/datastore: Allow prefaulting by mlockall()

While mlockall() is meant to lock page *memory*, effectively it will
also create and lock the corresponding page table entries.
Latency-sensitive applications expect not to experience any pagefaults
after calling mlockall(). However mlockall() ignores VM_IO mappings,
which is used by the generic vDSO datastore.
While the fault handler itself is very fast, going through the full
pagefault exception handling is much slower, on the order of 20us in a
test machine.

Since the memory behind the datastore mappings is always present and
accessible it is not necessary to use VM_IO for them.

The data page mapping is now also aligned with the architecture-specific
code pages. Some architecture-specific data pages, like the x86 VCLOCK
pages, continue to use VM_IO as they are not always mappable. They will
require their own special handling later when the general approach has
been agreed upon.

As a side-effect this will allow GUP on these pages and allow more ways
to access the data in them. This is fine, as all data in this mapping
is globally visible anyways. Either because it is mapped into all tasks,
or in the case of the time namespace pages, can be read from procfs.

Regular mlock() would also work, but userspace does not know the boundaries
of the vDSO.

Reported-by: Nam Cao <namcao@linutronix.de>
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Nam Cao <namcao@linutronix.de>
Link: https://patch.msgid.link/20260630-vdso-mlockall-v4-5-6c93708ce723@linutronix.de

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