ext4: hook the ext3 migration interface to the EXT4_IOC_SETFLAGS ioctl

This patch hooks the ext3 to ext4 migrate interface to
EXT4_IOC_SETFLAGS ioctl. The userspace interface is via chattr +e.  We
only allow setting extent flags.  Clearing extent flag (migrating from
ext4 to ext3) is not supported.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>