ffs: Add FFSV_REPLACE_DOOMED flag to ffs_vgetf()

It specifies that caller requests a fresh non-doomed vnode. If doomed
vnode is found in the hash, it should behave similarly to FFSV_REPLACE.
Or, to put it differently, the flag is same as FFSV_REPLACE, but only
when the found hashed vnode is doomed.

Reviewed by: chs, mkcusick
Tested by: pho
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation