scatterlist: inline sg_next()

sg_next() is a short function called frequently in I/O paths. Define it
in the header file so it can be inlined into its callers.

Link: https://lkml.kernel.org/r/20250416160615.3571958-1-csander@purestorage.com


Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Cc: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>