Don't clear _mounted_shares list in remoteFS while updating

This fix makes the updating of the _mounted_shares list in
remoteFS more of an atomic operation.  Previously this list
would be cleared, then rebuilt.  That allowed a race
condition where operations that took place during this
update would have a list of 0 shares to work with.

Closes-Bug: #1366083
cherry-picked from commit 9cc9cf111ef8a0265df5323fecaed12b0dabe312

Change-Id: I740d8f1b87db911242326a38ff398b81c5974cea