fs/fs.c: correctly interpret the '(max)len' parameter to fs_read()

The 'len' parameter passed to fs_read() actually denotes the maximum
number of bytes that fit into the callers buffer, not an expected
amount of data to be read.
Rename the parameter accordingly and honor the maxlen requested by the
caller appropriately.
Also remove the bogus warning message printed when the number of bytes
read is smaller than maxlen.