NFS: remove unneeded check in decode_devicenotify_args()

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Overflow check in not needed anymore after we switch to kmalloc_array().

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Fixes: a4f743a6 ("NFSv4.1: Convert open-coded array allocation calls to kmalloc_array()")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>