ext4: ignore errors when issuing discards

This is an effective revert of commit a30eec2a: "ext4: stop issuing
discards if not supported by device".  The problem is that there are
some devices that may return errors in response to a discard request
some times but not others.  (One example would be a hybrid dm device
which concatenates an SSD and an HDD device).

By this logic, I also removed the error checking from ext4's FITRIM
code; so that an error from a discard will not stop the FITRIM from
trying to trim the rest of the file system.

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