Credential might need to hang around longer than its parent and be used
outside of mnt_explock scope controlling netcred lifetime. Use separate
reference-counted ucred allocated separately instead.
While there, extend mnt_explock coverage in vfs_stdexpcheck and clean-up
some unused declarations in new NFS code.
Reported by: John Hickey
PR: kern/133439
Reviewed by: dfr, kib