fix snapshot content display with hide unreadable

fix snapshot content display with hide unreadable

With the hide unreadable option set, snapshots are be displayed
as empty because the shadow_copy2 module did not implement the
fget_nt_acl call that is used by the hide unreadable code and
so the paths were not corrected internally.

To prevent multiple conversions of the paths when the acl call
does a VFS_STAT (like the nfs4acl code does), a check was added
to convert_shadow2_name() so it will not touch paths any more
that look like they have already been converted.

Signed-off-by: Christian Ambach <christian.ambach@de.ibm.com>