mount.nfs: use nfs_error instead of fprintf in get_socket()

Use nfs_error() where appropriate.  I used "goto" here to reduce string
splitting and indenting past the point of readability.  Gee, it would be
nice if C had proper exception handling...

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Neil Brown <neilb@suse.de>