Add missing checks to zpl_xattr_* functions

Add missing checks to zpl_xattr_* functions

Linux specific zpl_* entry points, such as xattrs, must include
the same unmounted and sa handle checks as the common zfs_ entry
points. The additional ZPL_* wrappers are identical to their
ZFS_ counterparts except the errno is negated since they are
expected to be used at the zpl_ layer.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: John Gallagher <john.gallagher@delphix.com>
Closes #5866
Closes #7761