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