ksmbd: smb1: add buffer validation

Many functions assume the client sends well formed packets, or are
optimistic about the buffer length (using PATH_MAX or a fixed '256'
value) instead of the actual length.

The pattern followed to fix this boils down to extracting the total
buffer length and calculate how much data has been consumed.

Signed-off-by: Marios Makassikis <mmakassikis@freebox.fr>

Author: Marios Makassikis