MFV r268122:

4929 want prevsnap property

illumos/illumos-gate@b461c7460e5e77cf65f00151162e654220c6e2fb

MFC after:	2 weeks
Check if fchflags() is needed by fstat'ing before and check
the results.

Reviewed by:	jilles
X-MFC-With:	r267977