client: fix remaining Inode::put() caller, and make method psuedo-private

Not sure I can make this actually private and make Client::put_inode() a
friend method (making all of Client a friend would defeat the purpose).
This works well enough, though!

Signed-off-by: Sage Weil <sage@inktank.com>
client: send all request put's through put_request()

Make sure all MetaRequest reference put's go through the same path that
releases inode references, including all of the error paths.

Signed-off-by: Sage Weil <sage@inktank.com>