MFC r301745:

MFC r301745:

  Make sure the OSD methods for jail set and remove can't run concurrently,
  by holding allprison_lock exclusively (even if only for a moment before
  downgrading) on all paths that call PR_METHOD_REMOVE.  Since they may run
  on a downgraded lock, it's still possible for them to run concurrently
  with PR_METHOD_GET, which will need to use the prison lock.

git-svn-id: svn://svn.freebsd.org/base/stable/10@301907 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f