ext4: add debugging trigger for ext4_error()

Make it easy to test whether or not the error handling subsystem in
ext4 is working correctly.  This allows us to simulate an ext4_error()
by echoing a string to /sys/fs/ext4/<dev>/trigger_fs_error.

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