powerpc_mmu_radix: Release PTP reference on leaf ptpage allocation failure

0013741 fixed an edge case invloving mlock() and superpage creation
by creating and inserting a leaf pagetable page for mlock'd superpages.
However, the code does not properly release the reference to the
pagetable page in the error handling path.
This commit fixes the issue by adding calls to 'pmap_abort_ptp'
in the error handling path.

Reported by: alc
Approved by: markj (mentor)
Fixes: 0013741
Differential Revision: https://reviews.freebsd.org/D45582