Support statx() mask and query flags parameters

Support the query flags AT_STATX_FORCE_SYNC by forcing an attribute
revalidation, and AT_STATX_DONT_SYNC by returning cached attributes
only.

Use the mask to optimise away server revalidation for attributes
that are not being requested by the user.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>