EX-7786 test: get size before fallback to generic

EX-7786 test: get size before fallback to generic

If ll_file_seek() falls back to generic_file_llseek_size()
then make sure inode size is reliable

Test-Parameters: testlist=sanity env=ONLY=460f,ONLY_REPEAT=4 clientdistro=el9.1
Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: Ieb8c90e66fb19675ba41f0147c0d9cdaf29ea20a
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/51564
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>