ext4: add fallocate mode blocking for debugging purposes

ext4: add fallocate mode blocking for debugging purposes

If a particular fallocate mode is causing test failures, give the
tester the ability to block a particular fallocate mode so that the
use of a particular fallocate mode will be reported as not supported.

For example, if the COLLAPSE_RANGE fallocate mode is causing test
failures, this allows us to suppress it so we can more easily test the
rest of the file system code.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>