Store common Linux xattrs as native SA with xattr=sa

If xattr=sa is set, the following "security." and "system." xattrs are
not stored in the ZPL_DXATTR system attribute but instead are promoted
to native system attributes as follows:

          xattr                      System atrribute
    --------------------------------------------------------
    security.selinux            ZPL_SECURITY_SELINUX
    security.capability         ZPL_SECURITY_CAPABILITY
    system.posix_acl_access     ZPL_SYSTEM_POSIX_ACL_ACCESS
    system.posix_acl_default    ZPL_SYSTEM_POSIX_ACL_DEFAULT

These xattrs are natively used by the Linux kernel effectively as
extensions to an inode and, as such, should be managed atomically with
respect to other operations on their associated file.  For example,
when Selinux is enabled, a newly-created file should have the
"security.selinux" xattr set as an integral part of the file's creation.

Storing these xattrs as native system attributes allows for the ZPL to
more easily and naturally operate on them as an atomic part of other
operations.

Zdb will display these "native SA xattrs" under the new section heading of
"Native SA xattrs".

In order to save space in the dnode, the SPL_DXATTR SA is removed if it
becomes empty.