configfs: use capped length for ->store_attribute()

The difference between "count" and "len" is that "len" is capped at
4095.  Changing it like this makes it match how sysfs_write_file() is
implemented.

This is a static analysis patch.  I haven't found any store_attribute()
functions where this change makes a difference.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>