LU-18873 obdclass: add namelen check in encode_fn_len()

LU-18873 obdclass: add namelen check in encode_fn_len()

Add namelen check in function encode_fn_len() to avoid coverity
warnings.

Fixes: d097fcc51b8 ("LU-18810 obd: add a parameter to enable filename encoding")
Signed-off-by: Emoly Liu <emoly@whamcloud.com>
Change-Id: Ic1cc7238cd659254e286d8a595dc5203ae7dbc0b
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58608
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>