When digesting a file we always try toread PAGE_SIZE bytes. When we get a
short read because we reached the file end then the code works
correctly. If instead we only want to digest a part of the file then
we must make sure to only read up to 'size' bytes.

Fixes: b77582effd ("crypto: digest: Split memory vs. file code into separate functions")

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>