Both __d_lookup_rcu() and __d_lookup_rcu_op_compare() have the full
'name_hash' value of the qstr that they want to look up, and mask it off
to just the low 32-bit hash before calling down to d_hash().

Other callers just load the 32-bit hash and pass it as the argument.

If we move the masking into d_hash() itself, it simplifies the two
callers that currently do the masking, and is a no-op for the other
cases.  It doesn't actually change the generated code since the compiler
will inline d_hash() and see that the end result is the same.

[ Technically, since the parse tree changes, the code generation may not
  be 100% the same, and for me on x86-64, this does result in gcc
  switching the operands around for one 'cmpl' instruction. So not
  necessarily the exact same code generation, but equivalent ]

However, this does encapsulate the 'd_hash()' operation more, and makes
the shift operation in particular be a "shift 32 bits right, return full
word".  Which matches the instruction semantics on both x86-64 and arm64
better, since a 32-bit shift will clear the upper bits.

That makes the next step of introducing a "shift by runtime constant"
more obvious and generates the shift with no extraneous type masking.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>