init/do_mounts_rd.c: add EROFS initrd support

Initial RAM disk can be used together with FSDAX so that extra page
cache is no longer needed.

Compared to initramfs, it doesn't need to unpack files and dirs to
tmpfs one-by-one.  Instead, FSDAX-enabled images can be used directly.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>