dma-buf: Use task_lock instead of taking reference to file_struct

We take reference to files_struct while browsing files, putting
down the reference in put_file_struct could result in sleep when
close_file is called this results in a kernel BUG as sleep is not
allowed with in the critical section protected by read_lock. so,
remove both get_file_struct and put_file_struct calls instead use
task_lock while iterating through files as this lock protects the
file_struct.Also fix the thread handling code to properly iterate
over all the threads.

Change-Id: Ib432918a0355053c9f977fd72d6b99c60d2f6362
Signed-off-by: Vijayanand Jitta <vjitta@codeaurora.org>