fuse: Add support for shortcircuited read/write for files

Add support for shortcircuited read/write for files
when enabled through a userspace init option of
FUSE_PASSTHROUGH.

When FUSE_PASSTHROUGH is enabled all the reads and writes
to the fuse mount point go directly to the native filesystem
rather than through the fuse daemon. All requsts that aren't
read/write still go thought the userspace code.

This allows for significantly better performance on read and writes
and the difference between fuse and the native lower filesystem is
negligible.

Change-Id: If3bab9f776670426cb7801fba8151505a5b94a3c
Signed-off-by: Nikhilesh Reddy <reddyn@codeaurora.org>
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>