The tmpfs_link() must not dereference the filesystem-specific data for
a vnode until it is verified that the vnode indeed belongs to tmpfs
mount.  Otherwise, it might access random memory, at least in the
debug kernel.

Reported and tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Add some assertions for the code handling vm_object for tmpfs vnode.
In particular, vnode must be exclusively locked when the tmpfs vnode
and object are divorced.  When the vnode is opened, the object must be
still alive, since only live vnode can be opened, and the tmpfs node
owns a reference on the object.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Add convenience macro to assert tmpfs node lock.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
In tmpfs_alloc_file(), code after the 'out' label does only 'return
error;'.  Replace goto's with the return.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Extract the code to put a filesystem into the suspended state (at the
unmount time) in the helper vfs_write_suspend_umnt().  Use it instead
of two inline copies in FFS.

Fix the bug in the FFS unmount, when suspension failed, the ufs
extattrs were not reinitialized.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Style.  Add comment about lock mode.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Use tmpfs_vn_get_ino_gen() to handle the races with reclaim in tmpfs
dotdot lookup.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks