NFSv4.0: Fix a use-after-free problem in the asynchronous open()

In the Linux kernel, the following vulnerability has been resolved:

NFSv4.0: Fix a use-after-free problem in the asynchronous open()

Yang Erkun reports that when two threads are opening files at the same

time, and are forced to abort before a reply is seen, then the call to

nfs_release_seqid() in nfs4_opendata_free() can result in a

use-after-free of the pointer to the defunct rpc task of the other

thread.

The fix is to ensure that if the RPC call is aborted before the call to

nfs_wait_on_sequence() is complete, then we must call nfs_release_seqid()

in nfs4_open_release() before the rpc_task is freed.