mds: set want_base_dir to false for MDCache::discover_ino()

mds: set want_base_dir to false for MDCache::discover_ino()

When frozen inode is encountered, MDCache::handle_discover() sends
reply immediately if the reply message is not empty. When handling
"discover ino" requests, the reply message always contains the base
directory fragment. But requestor already has the base directory
fragment, the only effect of the reply message is wake the requestor
and make it send same "discover ino" request again. So the requestor
keeps sending "discover ino" requests but can't make any progress.

The fix is set want_base_dir to false for MDCache::discover_ino().
After set want_base_dir to false, also need update the code that
handles "discover ino" error.

This patch also remove unused error handling code for flag_error_dn

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>