ANDROID: virt: gunyah: Remove unbalanced folio_get

Page/folio pinning is handled directly by GUP. A lingering folio_get()
from guest_memfd upstream implementation was incorrectly preserved.
Remove it and fix the corresponding page leak.

Fixes: 296cceed ("ANDROID: virt: gunyah: Add gup based demand paging support")
Change-Id: I874cf9e07cf7bad20931071b0fc33e3b9c55ad8d
Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>