LU-9329 utils: add large xattr support for lustre_rsync.c

LU-9329 utils: add large xattr support for lustre_c

lustre_rsync.c had the problem of not able to get the desired
xattr buffer size, thus is not able to support large xattr
(> PATH_MAX).

lustre-rsync-test:1A would fail if /tmp file system on the
test client supports large xattr.

The following lustre_rsync log shows that lgetxattr keeps
on failing on large xattr (user.foo):

(trusted.lma,14307984) rc=0x18
lsetxattr(), rc=0, errno=0
(trusted.lov,14307984) rc=0x38
lsetxattr(), rc=0, errno=0
(trusted.link,14307984) rc=0x2f
lsetxattr(), rc=0, errno=0
(trusted.som,14307984) rc=0x18
lsetxattr(), rc=0, errno=0
(user.foo,14307984) rc=0xffffffff
(lustre.lov,14307984) rc=0x38
lsetxattr(), rc=-1, errno=95

Test-Parameters: trivial testlist=lustre-rsync-test
Signed-off-by: Li Xi <lixi@ddn.com>
Change-Id: I3ff49721b88dd31aa8af76da8932d5004c82ea09
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50997
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>