LU-11380 mdc: move empty xattr handling to mdc layer

LU-11380 mdc: move empty xattr handling to mdc layer

Extract duplicated logic around empty xattr handling from several
places in llite and consolidate it in mdc_getxattr().

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: I55b9653375a3cf2e6d9d3903743f2aa50ca2e322
Reviewed-on: https://review.whamcloud.com/33198
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>