virtiofsd: Fix fuse setxattr() API change issue

With kernel header updates fuse_setxattr_in struct has grown in size.
But this new struct size only takes affect if user has opted in
for fuse feature FUSE_SETXATTR_EXT otherwise fuse continues to
send "fuse_setxattr_in" of older size. Older size is determined
by FUSE_COMPAT_SETXATTR_IN_SIZE.

Fix this. If we have not opted in for FUSE_SETXATTR_EXT, then
expect that we will get fuse_setxattr_in of size FUSE_COMPAT_SETXATTR_IN_SIZE
and not sizeof(struct fuse_sexattr_in).

Fixes: 278f064e ("Update Linux headers to 5.13-rc4")
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Message-Id: <20210622150852.1507204-2-vgoyal@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>