ext2: Simplify partial sector access logic

Previously reading or writing zero full sectors (reading the end of
one sector and the beginning of the next for example) was special
cased and involved stack allocating a second sector buffer.  This
change uses the same code path for this case as well as when there
are a non-zero number of full sectors to access.  The result is
easier to read and reduces the maximum stack used.

Signed-off-by: Anton Staaf <robotboy@chromium.org>
Cc: Andy Fleming <afleming@freescale.com>
Acked-by: Detlev Zundel <dzu@denx.de>