Specify locking requirements with checklocks

[Ghanan]

checklocks simply checks for Lock/Unlock but doesn't distinguish between read/write locking requirements.

1. we may need write locks but checklocks passes with read locks (the following test is testing for wrong behaviour)

   gvisor/tools/checklocks/test/basics.go

   Lines 115 to 119 in 16b751b

    func testRWValidWrite(tc *rwGuardStruct) { 
    	tc.rwMu.RLock() 
    	tc.guardedField = 2 
    	tc.rwMu.RUnlock() 
    } 

2. A method may only need read OR write locks but checklocks does not yet support annotating it with that minimum requirement.

[Bhasker Hariharan]

I think there was problem with the generated SSA where it was hard to distinguish this use-case. I will have to revisit this to refresh my memory why we did not support this.

[Adin Scannell]

I don't think this is difficult anymore. I can knock this out soon.

[Tamir Duberstein]

Also the test right above it has the wrong name - it writes rather than reads.

gvisor/tools/checklocks/test/basics.go

Lines 109 to 113 in 16b751b

 func testRWValidRead(tc *rwGuardStruct) { 
 	tc.rwMu.Lock() 
 	tc.guardedField = 1 
 	tc.rwMu.Unlock() 
 }