libext2fs: call fsync(2) to clear stale errors for a new a unix I/O channel

libext2fs: call fsync(2) to clear stale errors for a new a unix I/O channel

Newer versions of Linux will retain errors and allow them to be
returned by fsync() or close(), even if the error happened before the
file descriptor was opened.  This was to accomodate Postgres's
"interesting" method of error collection.

Unfortunately, for e2fsck, we never want to see stale errors, as this
can cause xfstests (such generic/347) to fail with a false positive.

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