ANDROID: virt: gunyah: Correct n_mem_entries calculation in gunyah_gup_share_parcel

gunyah_gup_share_parcel incorrectly calculates n_mem_entries and places
items into the mem_entries array. To save number of bytes sent to RM, we
describe only folios -- not individual pages. We incorrectly filled the
mem_entries array with some entries as 0's. Track insertion into the
mem_entries array with new indexer: entries.

Bug: 358605784
Fixes: 296cceed ("ANDROID: virt: gunyah: Add gup based demand paging support")
Change-Id: I4738874579136a834bed86daba6d70e469512656
Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>