ext4: Make extents code sanely handle on-disk corruption

Add more run-time checking of extent header fields and remove BUG_ON
checks so we don't panic the kernel just because the on-disk filesystem
is corrupted.

Signed-off-by: Alex Tomas <alex@clusterfs.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>