scatterlist: fix extraneous '@'-sign kernel-doc notation

Using "@argname@" in kernel-doc produces "argname****" (with "argname" in
bold) in the generated html output, so use the expected kernel-doc
notation of just "@argname" instead.

"Fixes:" lines are added in case Matthew's patch [1] is backported.

Link: https://lkml.kernel.org/r/20250605002337.2842659-1-rdunlap@infradead.org
Link: https://lore.kernel.org/linux-doc/3bc4e779-7a79-42c1-8867-024f643a22fc@infradead.org/T/#m5d2bd9d21fb34f297aa4e7db069f09bc27b89007

 [1]
Fixes: 0db9299f ("SG: Move functions to lib/scatterlist.c and add sg chaining allocator helpers")
Fixes: 8d1d4b53 ("scatterlist: inline sg_next()")
Fixes: 18dabf47 ("Change table chaining layout")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>