inotify: start watch descriptor count at 1

The inotify_add_watch man page specifies that inotify_add_watch() will
return a non-negative integer.  However, historically the inotify
watches started at 1, not at 0.

Turns out that the inotifywait program provided by the inotify-tools
package doesn't properly handle a 0 watch descriptor.  In 7e790dd5

 we
changed from starting at 1 to starting at 0.  This patch starts at 1,
just like in previous kernels, but also just like in previous kernels
it's possible for it to wrap back to 0.  This preserves the kernel
functionality exactly like it was before the patch (neither method broke
the spec)

Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>