LU-11834 llite: fix temporary instance buffer size

The formatting of the cfg_instance variable was changed in LU-11809
to always use a fixed "%016llu" format, but the temporary buffer
allocations for the instance string were not changed to match the
printed value.

This results in string truncation in some situations.  Change the temp
buffer size to always have 16 bytes for the instance instead of using
sizeof(cfg_instance).

Fixes: cd294a1255 ("LU-11809 llite: don't use %p for cfg_instance")

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I5eca99afa2787cc57e739489b252b12af68cab07
Reviewed-on: https://review.whamcloud.com/33951
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>