ldb_controls: avoid unnecessary unchecked talloc_asprintf()s

ldb_controls: avoid unnecessary unchecked talloc_asprintf()s

The error paths when a control doesn't parse involved a lot of
talloc_asprintf()s and talloc_asprintf_append()s but almost no actual
printf formatting. The return values were not checked. This replaces
them with constant strings.

The one case that did use formatting looked like this:

 "invalid %s control syntax\n",  LDB_CONTROL_DIRSYNC_EX_NAME

and that has been replaced with

  "invalid dirsync_ex control syntax\n"

in line with the way it is done elsewhere.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>