FreeBSD: Remove HAVE_INLINE_FLSL use

These macros are deprecated in FreeBSD kernel for several years,
and unneeded for much longer. Instead, similar to Linux, let
kernel let compiler do the right things.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <alexander.motin@TrueNAS.com>
Closes #18004