- more fixes and improvements in GNS stuff:

  - small optimization in ll_gns_wait_for_mount(), changing return code semantics to
    usual 0 - ok, < 0 error.

  - handled possible nested mount with returning -ERETSARTSYS and causing endless loop
    of mounting.

  - check if found mount object is regular file to prevent from evil cases like it is
    suid bit marked directory, etc.

  - fix with completting all waiters for dentry mounting in progress.
  - added check for "." and ".." names in setting gns_object_name.
  - fixed default gns_object_name value.