If there is a corupted file system where the claimed depth of the
extent tree is -1, this can cause a massive buffer overrun leading to
sadness.

This addresses CVE-2018-10877.

https://bugzilla.kernel.org/show_bug.cgi?id=199417Signed-off-by: NTheodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org