nfs: enforce unique ILLEGAL opname when failed to match operation (el…

…astic#11503) (elastic#12025)

When we fail to map packet to a valid NFS4 operation, then opname
ILLEGAL with opcode it used, like `ILLEGAL (4294967295)`. While
this points to a bug in packet detection, such 'random' illegal
opnames create multiple new operations that confuse kibana.

Do not include opcode into opname ILLEGAL.

Signed-off-by: Tigran Mkrtchyan <[email protected]>
(cherry picked from commit f457715)