generic/631: use trusted.* extended attributes to filter NFS filesystem

generic/631: use trusted.* extended attributes to filter NFS filesystem

Since commit 2461e8901c ("generic/631: Add a check for extended
attributes"), check of user.* extended attributes was added to avoid
running this tests for filesystems that are not supported.

But it still fails on NFSv4.2 when mount overlayfs with the
following error.

/var/mnt/scratch/merged0: wrong fs type, bad option, bad superblock
on overlay, missing codepage or helper program, or other error.

dmesg log reports the following -
overlayfs: upper fs does not support xattr, falling back to index=off and metacopy=off.

As per the linux patch series(https://lwn.net/Articles/799185/ ), the
support of user extended attributes were added to NFSv4.2.  So this
case passed the check of extended attributes by _require_attrs.

As per the overlayfs documentation -

"The upper filesystem will normally be writable and if it is it must
support the creation of trusted.* and/or user.* extended attributes,
and must provide valid d_type in readdir responses, so NFS is not
suitable."

So we can replace user.* extended attributes check with trusted.* to
filter NFS filesystem.

Signed-off-by: Dai Shili <daisl.fnst@fujitsu.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>