Make /dev/zero reads interruptible by signals

This helps with bad latencies for large reads from /dev/zero, but might
conceivably break some application that "knows" that a read of /dev/zero
cannot return early.  So do this early in the merge window to give us
maximal test coverage, even if the patch is totally trivial.

Obviously, no well-behaved application should ever depend on the read
being uninterruptible, but hey, bugs happen.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>