nfs3_list_one_acl(): check get_acl() result with IS_ERR_OR_NULL

nfs3_list_one_acl(): check get_acl() result with IS_ERR_OR_NULL

There was a check for result being not NULL. But get_acl() may return
NULL, or ERR_PTR, or actual pointer.
The purpose of the function where current change is done is to "list
ACLs only when they are available", so any error condition of get_acl()
mustn't be elevated, and returning 0 there is still valid.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=81111
Signed-off-by: Andrey Utkin <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Fixes: 74adf83f5d77 (nfs: only show Posix ACLs in listxattr if actually...)
Cc: [email protected] # 3.14+
Signed-off-by: Trond Myklebust <[email protected]>