LU-10885 llite: enable flock mount option by default

LU-10885 llite: enable flock mount option by default

The "flock" mount option has been optional for many years, initially
because of potential stability issues, and also to provide a choice
for administrators to select between "flock" and "localflock" options.

However, from the large number of problems that users report when
trying to use applications that depend on this feature (typically
databases and other cloud stacks) that disabling flock by default
causes more problems than it solves.

Enable the "flock" (distributed coherent userspace locking) feature
by default.  If applications do not need this functionality, then it
will not affect them.  If applications *do* need this functionality,
they will get it.  If administrators really know what they are doing,
then they can use the "localflock" feature to enable client-local
flock functionality, possibly only on select nodes that need this.

Users wanting to disable this functionality should mount with the
existing "-o noflock" mount option, or build the client with the
"configure --disable-flock" option.

If clients are already using "-o {flock|localflock|noflock}" then
their existing options will be handled appropriately.

Lustre-change: https://review.whamcloud.com/32091
Lustre-commit: 3613af3e15cbc6091e3a16c8caeb1307be2d91f6

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I182637604fa22573b1da6b6b86d8915e3c3ebbe5
Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: Ben Evans <bevans@cray.com>
Reviewed-on: https://review.whamcloud.com/34987
Tested-by: Jenkins
Reviewed-by: Li Xi <lixi@ddn.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>