Fix sa_add_projid to lookup and update SA_ZPL_DXATTR (avoid DXATTR loss) (#16288)

sa_add_projid() gets called via zfs_setattr() for setting project id
on old file/dir, which were created before upgrading to project quota
feature. This function does lookup for all possible SA and update them
all together along with project ID at needed fixed offset. But its
missing lookup and update of SA_ZPL_DXATTR, effectively it losses
SA_ZPL_DXATTR.

Closes #16287
Signed-off-by: Jitendra Patidar <jitendra.patidar@nutanix.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Rob Norris <rob.norris@klarasystems.com>