LU-18740 mgs: size_t in contain_valid_fsname()

LU-18740 mgs: size_t in contain_valid_fsname()

to fix a build warning with gcc 11.5.0 (Rocky 9.3):

lustre/mgs/mgs_llog.c:4995:13: error: '__builtin_memcmp_eq'
 specified bound [18446744071562067968, 0] exceeds maximum
 object size 9223372036854775807 [-Werror=stringop-overread]
 4995 |         if (memcmp(buf, fsname, namelen) != 0)
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

Test-Parameters: trivial
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I77adc19e4d79d4a84a2cfe3c9601f5536ad8cc81
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58142
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>