staging: gasket: page table: remove extraneous memory barriers

Some explicit memory barriers in the page table code are not necessary,
either because:

(a) The barrier follows a non-relaxed MMIO access that already performs
a read or write memory barrier.

(b) The barrier follows DMA API calls for which the device-visible
effects of IOMMU programming are guaranteed to be flushed to the IOMMU
prior to the call returning, and doesn't need to sync with normal memory
access.

Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>