llist: make llist_add_batch() a static inline

The function is small enough that it should be, and it's a (very) hot path
for io_uring.  Doing this actually reduces my vmlinux text size for my
standard build/test box.

Before:
axboe@r7625 ~/g/linux (test)> size vmlinux
   text	   data	    bss	    dec	    hex		filename
19892174 5938310 2470432 28300916 1afd674	vmlinux

After:
axboe@r7625 ~/g/linux (test)> size vmlinux
   text	   data	    bss	    dec	    hex		filename
19891878 5938310 2470436 28300624 1afd550	vmlinux

Link: https://lkml.kernel.org/r/f1d104c6-7ac8-457a-a53d-6bb741421b2f@kernel.dk


Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>