Make BufferIsExclusiveLocked and BufferIsDirty work for local buffers.

Make BufferIsExclusiveLocked and BufferIsDirty work for local buffers.

These functions tried to check the state of the buffer's content lock
even for local buffers.  Since we don't use the content lock for a
local buffer, that would lead to a "false" result from
LWLockHeldByMeInMode, which would mean a misleading "false" answer
from BufferIsExclusiveLocked (we'd rather that case always return
"true") or an assertion failure in BufferIsDirty.

The core code never applies these two functions to local buffers,
and apparently no extensions do either, since we've not heard
complaints.  Still, in the name of future-proofing, let's fix
them to act as though a pinned local buffer is content-locked.

Author: Srinath Reddy <srinath2133@gmail.com>
Discussion: http://postgr.es/m/19396ef77f8.1098c4a1810508.2255483659262451647@zohocorp.com