With the introduction of software dirty bit emulation for managed mappings,
we should test ATTR_SW_DBM, not ATTR_AP_RW, to determine whether to set
PGA_WRITEABLE. In effect, we are currently setting PGA_WRITEABLE based on
whether the dirty bit is preset, not whether the mapping is writeable.
Correct this mistake.

Reviewed by: markj
X-MFC with: r350004
Differential Revision: https://reviews.freebsd.org/D21013