inline_fls: drop compile-time check

HAVE_INLINE_FLSLL is #defined always. This change assumes that where
__HAVE_INLINE_FLSLL is tested, the two leading underscores are a
mistake, and that the code will be better for using the efficient
flsll implementation.
Reviewed by: markj, mhorne
Differential Revision: https://reviews.freebsd.org/D40705