correct readlinkat(2) return type

r176215 corrected readlink(2)'s return type and the type of the last
argument. readlink(2) was introduced in r177788 after being developed
as part of Google Summer of Code 2007; it appears to have inherited the
wrong return type.

Man pages and header files were already ssize_t; update syscalls.master
to match.

PR: 197915
Submitted by: Henning Petersen <henning.petersen@t-online.de>
MFC after: 2 weeks