init/do_mounts_rd.c: fix NULL pointer dereference while loading initramfs

Make menuconfig allows one to choose compression format of an initial
ramdisk image.  But this choice does not result in duly compressed initial
ramdisk image.  Because - $ make install - does not pass on the selected
compression choice to the dracut(8) tool, which creates the initramfs
file.  dracut(8) generates the image with the default compression, ie.
gzip(1).

If a user chose any other compression instead of gzip(1), it leads to a
crash due to NULL pointer dereference in crd_load(), caused by a NULL
function pointer returned by the 'decompress_method()' routine.  Because
the initramfs image is gzip(1) compressed, whereas the kernel knows only
to decompress the chosen format and not gzip(1).

This patch replaces the crash by an explicit panic() call with an
appropriate error message. This shall prevent the kernel from
eventually panicking in: init/do_mounts.c: mount_block_root() with
  -> panic("VFS: Unable to mount root fs on %s", b);

[akpm@linux-foundation.org: mention that the problem is with the ramdisk, don't print known-to-be-NULL value]
Signed-off-by: P J P <prasad@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>