LU-1747 osd-zfs: Return zero when deleting nonexistent EAS

LU-1747 osd-zfs: Return zero when deleting nonexistent EAS

sanity 103 with ZFS targets fails:

[22] $ strace -o /tmp/tmo.strace cp -rp h i -- failed
 ~                                     ? cp: preserving permissions
for `i/x': No such file or directory
 ~                                     ? cp: preserving permissions
for `i': No such file or directory

The ENOENTs came from osd-zfs's osd_xattr_del(), when MDD tried to
delete "system.posix_acl_access".

Test-Parameters: envdefinitions=USE_OFD=yes,LOAD_MODULES_REMOTE=true
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Change-Id: I3736b1a89bec3174f7e0a6a848870adfa248d186
Reviewed-on: http://review.whamcloud.com/3833
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Wei <liwei@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>