list: introduce list_is_head() helper and re-use it in list.h

Introduce list_is_head() in the similar (*) way as it's done for
list_entry_is_head().  Make use of it in the list.h.

*) it's done as inliner and not a macro to be aligned with other
   list_is_*() APIs; while at it, make all three to have the same
   style.

Link: https://lkml.kernel.org/r/20211201141824.81400-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>