mesa: fix MSVC bitshift overflow warnings

mesa: fix MSVC bitshift overflow warnings

In the BITFIELD_MASK() macro, if b==32 the expression evaluates to
~0u, but the compiler still sees the expression (1 << 32) in the
unused part and issues a warning about integer bitshift overflow.

Fix that by using (b) % 32 to ensure the max shift is 31 bits.

This issue has been present for a while, but shows up much more
often because of the recent VBO changes.

Reviewed-by: Mathias Fröhlich <mathias.froehlich@web.de>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>