Use list_remove_head() where possible.

... instead of list_head() + list_remove(). On FreeBSD the list
functions are not inlined, so in addition to more compact code
this also saves another function call.

Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #14955