(2.14) [FIXED] Filestore delete map race

[MauriceVanVeen]

Fixes a data race introduced in #7387:

WARNING: DATA RACE
Read at 0x00c0002d17f0 by goroutine 527:
  github.com/nats-io/nats-server/v2/server/avl.(*SequenceSet).Range()
      server/avl/seqset.go:125 +0x3b
  github.com/nats-io/nats-server/v2/server.(*fileStore).deleteMap()
      server/filestore.go:10676 +0x164

Previous write at 0x00c0002d17f0 by goroutine 182:
  github.com/nats-io/nats-server/v2/server/avl.(*SequenceSet).Insert()
      server/avl/seqset.go:45 +0x6d
  github.com/nats-io/nats-server/v2/server.(*fileStore).PurgeEx()
      server/filestore.go:8626 +0x1b0c

Signed-off-by: Maurice van Veen github@mauricevanveen.com

[neilalexander]

LGTM