Support mounting same mount points with different fstab config

This commits allows multiple lines in a fstab have the same mount
point for first-stage mount, where the first successful mount will be
used.

This allows us to specify a common/shared fstab, with different
settings (e.g., ext4, squashfs) per line, to mount different types
of images on the same mount point. e.g., allows the device to mount
GSI (ext4) while support mounting the original squashfs system.img.

Sample fstab:
fstab in dt:
android {
    fstab {
        odm_ext4 {
            dev = "/dev/block/by-name/odm";
            mnt_point = "/odm";
            type = ext4;
            mnt_flags = "ro";
            fs_mgr_flags = "slotselect";
        };
        odm_f2fs {
            dev = "/dev/block/by-name/odm";
            mnt_point = "/odm";
            type = f2fs;
            mnt_flags = "ro";
            fs_mgr_flags = "slotselect";
        };
    };
}

fstab in file:
/dev/block/by-name/odm /odm ext4 ro slotselect
/dev/block/by-name/odm /odm f2fs ro slotselect

Test: make & boot & check /proc/mounts
Change-Id: I64892e3007b0cfd7d06dad1cfc664c45305197ee