fsx: Add check for mmap last-page zero fill

fsx: Add check for mmap last-page zero fill
Check to make sure the VM system provided 0's beyond the true end of
the file mapping (as required by mmap def in 1996 posix 1003.1)

This check was taken from the FreeBSD version of fsx.c.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>