tmpfs: for used pages, account really allocated pages, instead of file sizes
This makes tmpfs size accounting correct for the sparce files. Also
correct report st_blocks/va_bytes. Previously the reported value did not
accounted for the swapped out pages.
PR: 223015
Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D37097