Fix dump_fs mount point detection and fallback mount
    
    Simplify the code and fix mount point detection. The code logic is now
    much simpler: if $1 is not a mount point, call "mount --target $1" again
    to try mount it. "mount --target" cmd itself can handle all the /etc/fstab
    parsing job, so drop the buggy and complex bash code.
    
    Signed-off-by: Kairui Song <kasong@redhat.com>
    Acked-by: Pingfan Liu <piliu@redhat.com>