Step 4.2: start divorce of M_EXT and M_EXTPG

Step 4.2: start divorce of M_EXT and M_EXTPG

They have more differencies than similarities. For now there is lots
of code that would check for M_EXT only and work correctly on M_EXTPG
buffers, so still carry M_EXT bit together with M_EXTPG. However,
prepare some code for explicit check for M_EXTPG.

Reviewed by:	gallatin
Differential Revision:	https://reviews.freebsd.org/D24598