NFS: fs_context: validate UDP retrans to prevent shift out-of-bounds

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

NFS: fs_context: validate UDP retrans to prevent shift out-of-bounds

Fix shift out-of-bounds in xprt_calc_majortimeo(). This is caused by a garbage timeout (retrans) mount option being passed to nfs mount, in this case from syzkaller.

If the protocol is XPRT_TRANSPORT_UDP, then 'retrans' is a shift value for a 64-bit long integer, so 'retrans' cannot be >= 64. If it is >= 64, fail the mount and return an error.