Pick an ioctl number for EXT4_IOC_MIGRATE that won't conflict with
other ext4 ioctl's.  Since there haven't been any major userspace
users of this ioctl, we can afford to change this now, to avoid
potential problems later.

Also, reorder the ioctl numbers in ext4.h to avoid this sort of
mistake in the future.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>