any page faults from kernel mode are invalid if no
exception handler is defined for them, regardless whether mmap_sem is
held at the time of fault.

To easier detect code that can trigger the above error, add a check
also for the case where mmap_sem is acquired. As this has an overhead
make it a VM debug check.

Signed-off-by: Imre Deak <imre.deak@nokia.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
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