lists: add "const" qualifier to first arg of list_splice() operations

Since neither the list_splice() nor __list_splice() routines modify their
first argument, might as well declare them "const".

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>