unionfs: hold mount interlock while manipulating mnt_flag

This is for consistency with other filesystems.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273335 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Provide vfs suspension support only for filesystems which need it, take
two.

nullfs and unionfs need to request suspension if underlying filesystem(s)
use it. Utilize mnt_kern_flag for this purpose.

This is a fixup for 273271.

No strong objections from: kib
Pointy hat to: mjg
MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273336 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f