pseudofs: Ignore unsupported command in vop_setattr.

[johalun]

Summary

Users of pseudofs (e.g. lindebugfs), should be able to receive
input from command line via commands like "echo 1 > /path/to/file".
Currently this fails because sh tries to truncate the file first and
vop_setattr returns not supported error for this.

As per mailing list discussion, this patch simply ignores the
error and returns 0 instead.

Test Plan

Tested and this patch solves this issue for lindebugfs.