Unionfs: Grab the unionfs sb private data lock around branch info users

Unionfs: Grab the unionfs sb private data lock around branch info users

Locking/concurrency/race fixes.  Use the unionfs superblock rwsem, and grab
the read lock around every op that uses branch-related information, such as
branch counters.  Grab the write rwsem lock in operations which attempt to
change branch information, such as when adding/deleting branches.  This
will, for example, cause branch-management remount commands (which are
infrequent) to block a bit until all in-progress file operations on open
files are done.

Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>
[jsipek: whitespace fixes & more locks/unlocks]
Signed-off-by: Josef 'Jeff' Sipek <jsipek@cs.sunysb.edu>