backports: dissolve compat-3.9.h

backports: dissolve compat-3.9.h

Move everything into the appropriate header files.

Also include uidgid.h into the new fs.h since kernels
that had uidgid.h include it already, so it doesn't
change anything there, but those that don't have it
obviously can't have included it and people rely on
it behing included there.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>