staging/lustre/ldlm: Fix up LDLM_POOL_SYSFS_WRITER*_STORE define

The store method defined by LDLM_POOL_SYSFS_WRITER_STORE and
LDLM_POOL_SYSFS_WRITER_NOLOCK_STORE defines should
use size_t count, not unsigned long.

This produced a warning on i386 (and other 32bit architectures too,
I guess) where unsigned long is not 32 bit.

Reported by kbuild test bot.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>