nfsd: properly handle embedded newlines in fault_injection input

Currently rpc_pton() fails to handle the case where you echo an address
into the file, as it barfs on the newline. Ensure that we NULL out the
first occurrence of any newline.

Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>