list.h: add list_is_singular()

Add list_is_singular() to check a list has just one entry.

list_is_singular() is useful to check whether a list_head which have been
temporarily allocated for listing objects can be released or not.

Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>