vnode_pager_input: return runningbufspace back

Both vnode_pager_input_smlfs() and vnode_pager_generic_getpages()
increment runningbufspace, but also both delegate io completion handling
on the pbuf to either plain bdone() or filesystem-specific strategy
routine. Accidentally, for e.g. UFS it is g_vfs_strategy()/g_vfs_done().
The later calls bufdone() which handles runningbufspace reclamation.

For plain bdone() io done handler, nothing would return
accounted b_runningbufspace back. Do it in the new
helper vnode_pager_input_bdone(), as well as in
vnode_pager_generic_getpages_done() explicitly.

Note that potential multiple calls to runningbufwakeup() for the same
pbuf or buf completion are safe. runningbufwakeup() clears accounting
for the buffer, so second and later calls are nop.

The problem was found due to tarfs using small vnode pager input but not
g_vfs_strategy().

Reported by: des
Reviewed by: markj, sjg
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D39263