vm_map_protect may return an INVALID_ARGUMENT or PROTECTION_FAILURE
error response after clipping the first map entry in the region to be
reserved. This creates a pair of matching entries that should have
been "simplified" back into one, or never created. This change defers
the clipping of that entry until those two vm_map_protect failure
cases have been ruled out.

Reviewed by: alc
Approved by: markj (mentor)
Differential Revision: https://reviews.freebsd.org/D20711