libnfs.a: Fix API for getfh() & friends

libnfs.a: Fix API for getfh() & friends

This is more of a clean-up than a behavioral change.

POSIX requires that a "struct sockaddr" is the same size as a "struct
sockaddr_in".  Therefore, a variable or field of type "struct sockaddr"
cannot contain an AF_INET6 address.  However, "struct sockaddr *" is
often used to reference a generic (ie non-address family specific)
socket address, generating some confusion about this.

The nfsctl_arg struct uses a struct sockaddr (not a pointer) to pass
the client's IP address to the kernel.  This means the legacy nfsctl()
kernel API can never support IPv6.  Fortunately for us, this legacy
interface was replaced by a text-based cache interface a few years
back.  We don't need to support non-AF_INET addresses here.

The getfh() functions in nfs-utils provide a handy C API for the
kernel's nfsctl interface.  The getfh() functions still take a struct
sockaddr *, though, and that can imply that a non-IPv4 address can be
passed via this API.  To make it abundantly clear that only IPv4
addresses can be used with this interface, change the synopses of
getfh() and friends to take a struct sockaddr_in * instead of a struct
sockaddr * .

This makes these functions conform with other places in mountd and
exportfs that already grok the difference between a struct sockaddr
and a struct sockaddr_in.

While we're here...

Introduce some nice documenting comments for the get_fh() functions,
and...

Since mountd will support IPv6 in the near future, assert that the
family of client addresses passed to this API is indeed AF_INET, in
order to prevent non-AF_INET addresses from ever being passed to the
legacy nfsctl() interface.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>