tmpfs_getattr(): Return more correct allocated byte counts.

For VREG vnodes, return the resident page count (multiplied by PAGE_SIZE)
for the tmpfs node's anonymous VM object that stores actual file contents.

For all other vnodes, return the tmpfs_node's tn_size, which should not
be rounded to a page.

This change allows using stat(2) to identify a sparse file on tmpfs.

Reviewed by: kib
MFC after: 1 week