vfs: show_vfsstat: remove redundant initialization and check of error code

As err variable is now always checked right after each assignment, its
initialization is redundant and could be safely removed.  For the same
reason, the last check of err is also redundant and could be removed as
well.

Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>