[CIFS] clean up error handling in cifs_mount

Move all of the kfree's sprinkled in the middle of the function to the
end, and have the code set rc and just goto there on error. Also zero
out the password string before freeing it. Looks like this should also
fix a potential memory leak of the prepath string if an error occurs
near the end of the function.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>