nfs: don't report STATX_BTIME in ->getattr

NFS doesn't properly support reporting the btime in getattr (yet), but
61a968b4 mistakenly added it to the request_mask. This causes statx
for STATX_BTIME to report a zeroed out btime instead of properly
clearing the flag.

Cc: stable@vger.kernel.org # v6.3+
Fixes: 61a968b4

 ("nfs: report the inode version in getattr if requested")
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://bugzilla.redhat.com/show_bug.cgi?id=2214134
Reported-by: Boyang Xue <bxue@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>