LU-11108 mdt: propagate errors in mdt_getxattr()

LU-11108 mdt: propagate errors in mdt_getxattr()

In mdt_getxattr(), if mo_xattr_get() fails then return that error
value rather than letting mdt_nodemap_map_acl() mangle it.

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: I967bcc5ad6edf30b43f373e85f22fc922647c435
Reviewed-on: https://review.whamcloud.com/32743
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andrew Perepechko <c17827@cray.com>
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>