Goal : Avoid some locking/unlocking 'struct files_struct'->file_lock for mono 
threaded processes.

We define files_multithreaded() function .

static inline int files_multithreaded(const struct files_struct *files)
{
        return sizeof(files->file_lock) > 0 && atomic_read(&files->count) > 1;
}

On plain UP kernel (not preemptable nor spinlock debug), this function is a 
const 0, so that gcc can wipe out some code.

On preemptible or SMP, or spinlock debug kernels, the result is true only if 
the ref count is greater than 1 (multi threaded process or /proc/{pid}/fd is 
under investigation by another task)

This patch increases kernel size but pros are worth the cons, as said Linus 
himself, we should increase performance of mono-threaded tasks....

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>