staging: ramster: Dont build ramster when CONFIGFS_FS=m

Ramster can't be a module (yet) and depends on CONFIGFS_FS=y, but
allmodconfig builds with CONFIGFS_FS=m, which breaks the build.
And forcing CONFIGFS_FS=y with select breaks the build in other ways.
So just don't build ramster unless CONFIGFS_FS=y.

Also, while we're here, add a comment as to why BROKEN is depended.

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>