LU-10264 misc: fix possible array overflow

LU-10264 misc: fix possible array overflow

Fix a static analysis error.

lustre/obdclass/obd_mount_server.c:1830 in osd_start(), buffer
    flagstr has size 16 but length of format string "%lu:%lu" is 31.
Increase the size of buffer to hold maximal-sized strings plus NUL.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I3cc80d66bbb537161a561f4f2ba7830dde2cab07
Reviewed-on: https://review.whamcloud.com/32242
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>