TestOnDiskFull_Concurrent failed under race

[RaduBerinde]

https://github.com/cockroachdb/pebble/actions/runs/12590509285/job/35092160722?pr=4210

--- FAIL: TestOnDiskFull_Concurrent (0.03s)
    disk_full_test.go:129: 
        	Error Trace:	/home/runner/work/pebble/pebble/vfs/disk_full_test.go:129
        	            				/opt/hostedtoolcache/go/1.23.4/x64/src/runtime/asm_amd64.s:1700
        	Error:      	Received unexpected error:
        	            	uh oh: mock mock: no space left on device
        	            	(1) attached stack trace
        	            	  -- stack trace:
        	            	  | github.com/cockroachdb/pebble/vfs.(*enospcMockFS).maybeENOSPC
        	            	  | 	/home/runner/work/pebble/pebble/vfs/disk_full_test.go:158
        	            	  | github.com/cockroachdb/pebble/vfs.(*enospcMockFS).Create
        	            	  | 	/home/runner/work/pebble/pebble/vfs/disk_full_test.go:164
        	            	  | github.com/cockroachdb/pebble/vfs.(*enospcFS).Create
        	            	  | 	/home/runner/work/pebble/pebble/vfs/disk_full.go:197
        	            	  | github.com/cockroachdb/pebble/vfs.TestOnDiskFull_Concurrent.func2
        	            	  | 	/home/runner/work/pebble/pebble/vfs/disk_full_test.go:127
        	            	  | runtime.goexit
        	            	  | 	/opt/hostedtoolcache/go/1.23.4/x64/src/runtime/asm_amd64.s:1700
        	            	Wraps: (2) uh oh
        	            	Wraps: (3) mock mock
        	            	Wraps: (4) no space left on device
        	            	Error types: (1) *withstack.withStack (2) *errutil.withPrefix (3) *fs.PathError (4) syscall.Errno
        	Test:       	TestOnDiskFull_Concurrent
    disk_full_test.go:136: 
        	Error Trace:	/home/runner/work/pebble/pebble/vfs/disk_full_test.go:136
        	Error:      	Not equal: 
        	            	expected: 0x14
        	            	actual  : 0x13
        	Test:       	TestOnDiskFull_Concurrent
FAIL
FAIL	github.com/cockroachdb/pebble/vfs	1.6[97](https://github.com/cockroachdb/pebble/actions/runs/12590509285/job/35092160722?pr=4210#step:4:98)s

Jira issue: PEBBLE-318