ARM: Add support for checking access permissions on prefetch aborts

ARMv6 introduces non-executable mappings, which can cause prefetch aborts
when an attempt is made to execute from such a mapping.  Currently, this
causes us to loop in the page fault handler since we don't correctly
check for proper permissions.

Fix this by checking that VMAs have VM_EXEC set for prefetch aborts.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>