ext4: replace BUG() with return -EIO in ext4_ext_get_blocks

This patch fixes the Kernel BZ #14286.  When the address of an extent
corresponding to a valid block is corrupted, a -EIO should be reported
instead of a BUG().  This situation should not normally not occur
except in the case of a corrupted filesystem.  If however it does,
then the system should not panic directly but depending on the mount
time options appropriate action should be taken. If the mount options
so permit, the I/O should be gracefully aborted by returning a -EIO.

http://bugzilla.kernel.org/show_bug.cgi?id=14286

Signed-off-by: Surbhi Palande <surbhi.palande@canonical.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>