Change synchronized on FileSystemContext to avoid deadlock #15766

Job	Run time
modules: (alluxio.client.cli.,!alluxio.client.cli.fs.)	1s
modules: (alluxio.client.cli.fs.**)	1s
modules: (alluxio.client.fs.,!alluxio.client.fs.concurrent.,!alluxio.client.fs.io.**)	1s
modules: (alluxio.client.fs.concurrent.,alluxio.client.fs.io.)	1s
modules: (alluxio.client.,!alluxio.client.fs.,!alluxio.client.cli.,!alluxio.client.fuse.)	1s
modules: (alluxio.job.,alluxio.master.,alluxio.stress.**)	1s
modules: (alluxio.server.ft.,!alluxio.server.ft.journal.raft.,!alluxio.server.ft.journal.ufs...	1s
modules: (alluxio.server.ft.journal.raft.,alluxio.server.ft.journal.ufs.)	1s
modules: (alluxio.server.,!alluxio.server.ft.)	1s
	9s

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