volume perf plugin failed when flexvol naming has __000x

[Hardikl]

Thanks to florent4155 for raising on Discord.

Due to different naming of flexvol, it's not guarantee to decide the volume is flexgroup_constituent or flexvol just based on __000x regex, which eventually creating duplicate instance error and later nil pointer exception due to invalid map access as shown in below.

This is one of the volume which is flexvol: TR0_P04_0010_pol_svm41_poll_vf257_v07_Backup__0001

[cgrinds]

Verified in 24.11 ddb97c5
florent4155 also verified in an earlier nightly build