Fix "multiple definition of `debugfs_create_size_t'"

Introduced by 8adb711f ("debugfs:
introduce stub for debugfs_create_size_t() when DEBUG_FS=n") and due to
a simple missing "static inline".

Reported-and-tested-by: Jeff Chua <jeff.chua.linux@gmail.com>
Acked-by: Greg KH <gregkh@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>