mm/shmem.c: Use new form of *@param in kernel-doc

Use the form of *@param which kernel-doc recognizes now.
This resolves the warnings from "make htmldocs" as reported in [1].

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: [1] https://lore.kernel.org/r/20240223153636.41358be5@canb.auug.org.au/


Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>