Return correct disk free/available size when FS is mounted with size = 0

When tmpfs is mounted with size = 0 (i.e. without any limit), then statvfs
will return 0 for f_bfree, f_bavail, and f_blocks. Catch this case and return
max_int64 instead.

BUG=628710

Review-Url: https://codereview.chromium.org/2152283003
Cr-Commit-Position: refs/heads/master@{#406473}
1 file changed