misc: uid_stat: Include linux/atomic.h instead of asm/atomic.h

Building the uid_stat driver on sparc32 fails with the following errors.

include/linux/atomic.h: In function 'atomic_add_unless':
include/linux/atomic.h:437:2: error:
	implicit declaration of function '__atomic_add_unless'
include/linux/atomic.h: In function 'atomic_andnot':
include/linux/atomic.h:454:2: error:
	implicit declaration of function 'atomic_and'
include/linux/atomic.h: In function 'atomic_set_mask':
include/linux/atomic.h:465:2: error:
	implicit declaration of function 'atomic_or'
include/linux/atomic.h: In function 'atomic_inc_not_zero_hint':
include/linux/atomic.h:490:3: error:
	implicit declaration of function 'atomic_cmpxchg'
include/linux/atomic.h: In function 'atomic_dec_if_positive':
include/linux/atomic.h:537:2: error:
	implicit declaration of function 'atomic_read'

Fixes: 6b6d5fbf ("misc: uidstat: Adding uid stat driver to collect network statistics.")
Signed-off-by: Guenter Roeck <groeck@chromium.org>