LU-13721 utils: fix 'lfs find --pool' for PFL files

LU-13721 utils: fix 'lfs find --pool' for PFL files

Fix "lfs find --pool" to check the lov_user_md_v3 for the specified
pool name in find_check_pool() for composite files (PFL or FLR).
The v3 pointer was initialized from the main layout xattr, but was
not being refreshed for each of the components in the file.

Add a test case for "lfs find --pool" usage.

Fixes: 5a76aee24476 ("LU-8998 lfs: user space tools for PFL")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I6b6f21fd2fdf58f46972704cb6fb425a943ebbe5
Reviewed-on: https://review.whamcloud.com/39196
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>