e4defrag: return more specific error message on ioctl failure

e4defrag: return more specific error message on ioctl failure

Currently e4defrag relies on the EXT4_IOC_MOVE_EXT ioctl to perform online
defragmentation. However, this iotcl kernel patch is not available before
2.6.30-rc1. e4defrag shall fail without obvious reasons on systems running
older kernels. The patch adds more detailed error message addressing this
issue and prompts users with the minimal kernel version that is needed to
run e4defrag.

Signed-off-by: Peng Tao <bergwolf@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>