idr: add ida_is_empty

Two of the USB Gadgets were poking around in the internals of struct ida
in order to determine if it is empty.  Add the appropriate abstraction.

Link: http://lkml.kernel.org/r/1480369871-5271-63-git-send-email-mawilcox@linuxonhyperv.com
Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Acked-by: Konstantin Khlebnikov <koct9i@gmail.com>
Tested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Felipe Balbi <balbi@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Michal Nazarewicz <mina86@mina86.com>
Cc: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>