[linux-cifs-client] turning off posix advisory locks and falling back to mandatory locks

[Steve French]

If we remove the check on (roughly) line 647 of fs/cifs/file.c for the 
experimental flag (/proc/fs/cifs/Experimental) - we will do posix style 
advisory locking (remotely) to Samba and any other current servers which 
implement the POSIX protocol extensions to cifs - rather than map to 
mandatory locks as before.  This is the correct default behavior now 
that the code has been out there a while and tests out ok.

But would this ever need to be turned off (ie different options than are 
available now)?

    1) default
          - POSIX advisory locking if server claims support for 
CIFS_UNIX_FCNTL_CAP in its CIFS Unix Extensions capability
          - otherwise map advisory locks to mandatory locks
    2) if "nobrl" mount option, the client only sets/gets/enforces byte 
range locks locally (does not send them to the server) which is the best 
we can do for some important (albeit oddly coded) apps (which e.g. use 
different file handles to write than the one they used to get the lock) 
when run to Windows servers
    3) turn off Unix extensions (/proc/fs/cifs/LinuxExtensionsEnabled) 
which has the effect of turning off posix byte range locking, but also 
symlinks and a few other things only supported when the Unix Extensions 
are negotiated

Is there a need for another cifs mount option (to allow someone to turn 
off posix advisory locking on the mount - and use manadotory - but still 
use the rest of the CIFS Unix/POSIX protocol extensions?     Although 
there is an sb flag MS_MANDLOCK (which mount.cifs sets), I don't think 
that using MS_MANDLOCK helps much - and we should probably turn it off 
when the server supports mandatory locks.