folly/fibers/TimedMutex-inl.h: avoid shadowing warnings
folly/fibers/TimedMutex-inl.h: avoid shadowing warnings

Summary:
Fix a warning exposed by gcc's -Wshadow-compatible-local.

Rename inner "lockStolen" to "stolen".

Differential Revision: D4346385

fbshipit-source-id: 323cb6c96ecfe18b5540525f4124dbd75faf0b1b

charsyam/folly,rklabs/folly,rklabs/folly,Orvid/folly,floxard/folly,charsyam/folly,charsyam/folly,floxard/folly,floxard/folly,Orvid/folly,facebook/folly,Orvid/folly,rklabs/folly,Orvid/folly,rklabs/folly,facebook/folly,rklabs/folly,floxard/folly,floxard/folly,Orvid/folly,facebook/folly,facebook/folly,facebook/folly,charsyam/folly