vfs: conditionally clear close-on-exec flag

We clear the close-on-exec flag when opening and closing files, and the
bit was almost always already clear before.  Avoid dirtying the
cacheline if the clearning isn't necessary.  That avoids unnecessary
cacheline dirtying and bouncing in multi-socket environments.

Eric Dumazet has a file descriptor benchmark that goes 4% faster from
this on his two-socket machine.  It's probably partly superlinear
improvement due to getting slightly less spinlock contention on the
file_lock spinlock due to less work in the critical section.

Tested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/file.c

@@ -236,7 +236,8 @@ static inline void __set_close_on_exec(int fd, struct fdtable *fdt)
 
 static inline void __clear_close_on_exec(int fd, struct fdtable *fdt)
 {
-	__clear_bit(fd, fdt->close_on_exec);
+	if (test_bit(fd, fdt->close_on_exec))
+		__clear_bit(fd, fdt->close_on_exec);
 }
 
 static inline void __set_open_fd(unsigned int fd, struct fdtable *fdt)