Support special mount information via "dracut_args"
    
    There are some complaints about nfs kdump that users must mount
    nfs beforehand, which may cause some overhead to nfs server.
    For example, there're thounsands of diskless clients deployed with
    nfs dumping, each time the client is boot up, it will trigger
    kdump rebuilding so will mount nfs, thus resulting in thousands
    of nfs request concurrently imposed on the same nfs server.
    
    We introduce a new way of specifying mount information via the
    already-existent "dracut_args" directive(so avoid adding extra
    directives in /etc/kdump.conf), we will skip all the filesystem
    mounting and checking stuff for it. So it can be used in the
    above-mentioned nfs scenario to avoid severe nfs server overhead.
    
    Specifically, if there is any "--mount" information specified via
    "dracut_args" in /etc/kdump.conf, always use it as the final mount
    without any validation(mounting or checking like mount options,
    fs size, etc), so users are expected to ensure its correctness.
    
    NOTE:
    -Only one mount target is allowed using "dracut_args" globally.
    -Dracut will create <mountpoint> if it doesn't exist in kdump kernel,
     <mountpoint> must be specified as an absolute path.
    -Users should do a test first and ensure it works because kdump does
     not prepare the mount or check all the validity.
    
    Reviewed-by: Pratyush Anand <panand@redhat.com>
    Suggested-by: Dave Young <dyoung@redhat.com>
    Acked-by: Dave Young <dyoung@redhat.com>
    Signed-off-by: Xunlei Pang <xlpang@redhat.com>