osdc: remove implicit LingerOp reference between watch/unwatch

osdc: remove implicit LingerOp reference between watch/unwatch

before this change set, linger_register() returned a raw LingerOp
pointer with an implicit reference for the caller. for librados,
this implicit reference is only dropped when the corresponding
unwatch() calls linger_cancel()

after commit 94f42b648feea77bd09dc3fdb48e6db2b48c7717 introduced
linger_by_cookie(), unwatch() no longer has a safe way to drop this
implicit reference. to prevent LingerOp leaks when unwatch() returns
ENOTCONN, we can't hold this implicit reference count until unwatch()

linger_register() now returns an explicit reference to the caller as
intrusive_ptr<LingerOp>. this helps to guarantee that this reference
count gets dropped before the completion of watch()/aio_watch()

because linger_register() no longer acquires an implicit reference for
the caller, linger_cancel() no longer drops it with info->put()

Reported-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Casey Bodley <cbodley@redhat.com>