libnetlink.c, ss.c: properly handle fread() errors

libnetlink.c, ss.c: properly handle fread() errors

fread(3) returns size_t data type which is unsigned, thus check
`if (fread(...) < 0)' is always false. To check if fread(3) has
failed, user should check error indicator with ferror(3).

This commit also changes read logic a little bit by being less
forgiving for errors. Previous logic was checking if fread(3)
read *at least* required ammount of data, now code checks if
fread(3) read *exactly* expected ammount of data. This makes
sense because code parses very specific binary file, and reading
even 1 less/more byte than expected, will later corrupt data anyway.

Signed-off-by: Michał Łyszczek <michal.lyszczek@bofc.pl>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>