gfs2: Fix gfs2_file_buffered_write endless loop workaround

Since commit 554c577c, gfs2_file_buffered_write() can accidentally
return a truncated iov_iter, which might confuse callers.  Fix that.

Fixes: 554c577c

 ("gfs2: Prevent endless loops in gfs2_file_buffered_write")
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>