lib/string.c: fix kernel-doc warnings

Fix kernel-doc warnings (@arg name) in string.c::skip_spaces().

  Warning(lib/string.c:347): No description found for parameter 'str'
  Warning(lib/string.c:347): Excess function parameter 's' description in 'skip_spaces'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>