Remove debug.zfs_flags in favor of the new vfs.zfs.debug_flags.
Replace TUNABLE_INT with CTLFLAG_RWTUN.

Submitted by:	avg (debug.zfs_flags removal), smh (TUNABLE_INT replacement)


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271798 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f