pseudofs: Ignore unsupported commands in vop_setattr.

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. This patch simply
ignores the error and returns 0 instead.

Reviewed by: imp (mentor), asomers
Approved by: imp (mentor), asomers
MFC after: 1 week
Differential Revision: D20451