fuse: Use BUG_ON to check pos validity in fuse_aio_write

Change the check that iocb->ki_pos should be equal to pos
to a BUG_ON from a WARN_ON to ensure we actually catch
the bug before it propagates down and causes other errors.

Change-Id: Ie6d537f3182f638a50166b456226e9c10bb1e3d4
Signed-off-by: Nikhilesh Reddy <reddyn@codeaurora.org>