[ARM] Resolve fuse and direct-IO failures due to missing cache flushes

fuse does not work on ARM due to cache incoherency issues - fuse wants
to use get_user_pages() to copy data from the current process into
kernel space.  However, since this accesses userspace via the kernel
mapping, the kernel mapping can be out of date wrt data written to
userspace.

This can lead to unpredictable behaviour (in the case of fuse) or data
corruption for direct-IO.

This resolves debian bug #402876

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