cifs: avoid extra calls in posix_info_parse

In posix_info_parse() we call posix_info_sid_size twice for each of the owner and the group
sid. The first time to check that it is valid, i.e. >= 0 and the second time
to just pass it in as a length to memcpy().
As this is a pure function we know that it can not be negative the second time and this
is technically a false warning in coverity.
However, as it is a pure function we are just wasting cycles by calling it a second time.
Record the length from the first time we call it and save some cycles as well as make
Coverity happy.

Addresses-Coverity-ID: 1491379 ("Argument can not be negative")

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>