avoid extra copy of data buffer in nfs3.read

[radai-rosenblatt]

for the case where # of bytes read == count the copy is definitely not required. i dont know if nfs allows returning a buffer larger than the amount of actual data in it (on the one hand there is a count field in the response, on the other the rfc says nothing) - so i left the copy in place for those cases. however, i think that bytesRead==count should be fairly common for files larger then the max buffer size (32k by default).

i've also removed an unused UnixUser.

[dcache-ci]

Can one of the admins verify this patch?

[kofemann]

ok to test.

[dcache-ci]

Test PASSed.