locks: print a warning when mount fails due to lack of "mand" support

Since 9e8925b6 ("locks: Allow disabling mandatory locking at compile
time"), attempts to mount filesystems with "-o mand" will fail.
Unfortunately, there is no other indiciation of the reason for the
failure.

Change how the function is defined for better readability. When
CONFIG_MANDATORY_FILE_LOCKING is disabled, printk a warning when
someone attempts to mount with -o mand.

Also, add a blurb to the mandatory-locking.txt file to explain about
the "mand" option, and the behavior one should expect when it is
disabled.

Reported-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jeff Layton <jlayton@kernel.org>