Pass the buffer size to the file detection code. This makes sure we do not
read past the buffer. This is especially useful for ext filesystem detection
as the magic is at byte offset 1080. Also introduce a FILE_TYPE_SAFE_BUFSIZE
define which is set to the minimum bufsize the detection code needs to detect
all known filetypes.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>