err.h: IS_ERR() can accept __user pointers

Sparse generates a false positive when you pass a __user or __iomem
pointer to the IS_ERR() functions.

  drivers/rtc/rtc-ds1286.c:344:36: sparse: incorrect type in argument 1 (different address spaces)
  drivers/rtc/rtc-ds1286.c:344:36:    expected void const *ptr
  drivers/rtc/rtc-ds1286.c:344:36:    got unsigned int [noderef] [usertype] <asn:2>*rtcregs

We can silence these by adding a __force here and upgrading to Sparse
v0.4.5-rc1 or later.

This change has no effect when using current Sparse releases.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Christopher Li <sparse@chrisli.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>