ext4: remove while loop in mpage_map_and_submit_extent()

ext4: remove while loop in mpage_map_and_submit_extent()

The while loop in mpage_map_and_submit_extent() is pointless; this
function is only called if mpd.map.m_len is non-zero, and at the end
of the while loop, mpage_map_and_submit_buffers sets mpd.map.m_len to
be zero on success.  (And on a failure, we return out of the while
loop anyway.)

Hence, the body of the while loop is guaranteed to execute once and
exactly once.  This also fixes a gcc warning complaining that err
might be left uninitialized.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Jan Kara <jack@suse.cz>