btrfs: add a NOSPACECACHE mount option flag

With the old mount API we'd pre-populate the mount options with the
space cache settings of the file system, and then the user toggled them
on or off with the mount options.  When we switch to the new mount API
the mount options will be set before we get into opening the file
system, so we need a flag to indicate that the user explicitly asked for
-o nospace_cache so we can make the appropriate changes after the fact.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>