LU-15420 uapi: avoid gcc-11 -Werror=stringop-overread

LU-15420 uapi: avoid gcc-11 -Werror=stringop-overread

GCC 11 warns about string and memory operations on fixed address:

In function ‘strlen’,
    inlined from ‘changelog_rec_sname’ at include/uapi/linux/lustre/lustre_user.h:1981:19,
    inlined from ‘mdd_changelog_rec_ext_rename’ at lustre/mdd/mdd_dir.c:932:2,
    inlined from ‘mdd_changelog_ns_store’ at lustre/mdd/mdd_dir.c:1061:3:
include/linux/fortify-string.h:25:33: error: ‘__builtin_strlen’
reading 1 or more bytes from a region of size 0 [-Werror=stringop-overread]
   25 | #define __underlying_strlen     __builtin_strlen

The reason is that we are looking for an address right after the end
of the chanelog record header which gcc thinks is an overrun. Rework
the code to allow us to index the memory right after the changelog
record header.

Also fix a long hidden bug in the lustre snmp code.

Change-Id: I13479b9074a392330d39f01656b26f9e9a91a8ec
Signed-off-by: James Simmons <jsimmons@infradead.org>