fsx: allow zero range operations to cross eof

fsx: allow zero range operations to cross eof

Currently we are limiting the range for zero range operations to stay
within the i_size boundary. This is not optimal because like this we lose
coverage of the filesystem's zero range implementation, since zero range
operations are allowed to cross the i_size. Fix this by limiting the range
to 'maxfilelen' and not 'file_size', and update the 'file_size' after each
zero range operation if needed.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>