mount.nfs: make nfs_options2pmap return errors

mount.nfs: make nfs_options2pmap return errors

Up until now, nfs_options2pmap() has been passed mount options that
have already gone through the kernel's parser successfully.  So, it
never had to check for invalid mount option values.

However, we are about to pass it options that come right from the
user.  So nfs_options2pmap() will now need to report an error and
fail if it encounters a bogus value for any of the options it cares
about.

 =====

Note that nfs_options2pmap() will allow a bogus value for an option
if the same option is specified farther to the right with a useable
value.

For example, if a user specifies "proto=foo,...,tcp" then
nfs_options2pmap() uses "tcp" and ignores "proto=foo".

However, if the options are specified in the other order:
"tcp,...,proto=foo" then nfs_options2pmap() will fail.  This is a simple
and unambiguous extension of the "rightmost wins" rule.

Since mount.nfs strips out these options out and replaces them with
the rpcbind-negotiated options before invoking mount(2), the kernel
should never receive bogus values for these options from mount.nfs in
such cases.

This is probably slightly more flexible behavior than the legacy
mount implementation, but should be harmless.  All mount options
unrelated to pmap are ignored by nfs_options2pmap().

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