The fragment lookup should only be performed, when
at least one of the fragment flags are set.

This change was deliberately not included in commit
68aba004 ("net: sparx5: flower: fix fragment flags handling")
as it's only needed for future proffing the code, since
"mask" is currently only set in conjunction with the
fragment flags.
(The 3rd flag FLOW_DIS_ENCAPSULATION is only used with "key")

Only compile tested.
Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Reviewed-by: Daniel Machon <daniel.machon@microchip.com>
Tested-by: Daniel Machon <daniel.machon@microchip.com>
Link: https://lore.kernel.org/r/20240424121632.459022-2-ast@fiberby.netSigned-off-by: Jakub Kicinski <kuba@kernel.org>