Unionfs: Change the semantics of sb info's rwsem

Unionfs: Change the semantics of sb info's rwsem

This rw semaphore is used to make sure that a branch management
operation...

1) will not begin before all currently in-flight operations complete

2) any new operations do not execute until the currently running branch
management operation completes

Reworked the patch a bit, added comments, and fixed some bugs, from the
version originally committed into the master branch.

TODO: rename the functions unionfs_{read,write}_{,un}lock() to something
more descriptive.

Signed-off-by: Josef 'Jeff' Sipek <jsipek@cs.sunysb.edu>
Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>