net/tftp: Fix stack buffer overflow in tftp_open()

net/tftp: Fix stack buffer overflow in tftp_open()

An overly long filename can be passed to tftp_open() which would cause
grub_normalize_filename() to write out of bounds.

Fixed by adding an extra argument to grub_normalize_filename() for the
space available, making it act closer to a strlcpy(). As several fixed
strings are strcpy()'d after into the same buffer, their total length is
checked to see if they exceed the remaining space in the buffer. If so,
return an error.

On the occasion simplify code a bit by removing unneeded rrqlen zeroing.

Reported-by: B Horn <b@horn.uk>
Signed-off-by: B Horn <b@horn.uk>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>