vfs: stop unlocking the vnode upfront in vput

[mjg]

Summary

Doing so runs into races with filesystems which make half-constructed vnodes visible to other users, while depending on the chain vput -> vinactive -> vrecycle to be executed without dropping the vnode lock.

Impediments for making this work got cleared up (notably vop_unlock_post now does not do anything and lockmgr stops touching the lock after the final write). Stacked filesystems keep vhold/vdrop across unlock, which arguably can now be eliminated.

[jeff]

thank you

head/sys/kern/vfs_subr.c

#
     SUBDIR += py-pyhs
     SUBDIR += py-pylibmc
     SUBDIR += py-pymssql
     SUBDIR += py-pymysql
     SUBDIR += py-pyodbc
     SUBDIR += py-pypuppetdb
     SUBDIR += py-pypuppetdb03
     SUBDIR += py-pytc
+    SUBDIR += py-python-arango
     SUBDIR += py-python-memcached
     SUBDIR += py-python-rrdtool
     SUBDIR += py-python-sql
     SUBDIR += py-python-swiftclient
     SUBDIR += py-qt4-sql
     SUBDIR += py-qt5-sql
     SUBDIR += py-queries
     SUBDIR += py-rb