btrfs-progs: Use sysconf instead of getpagesize

btrfs-progs: Use sysconf instead of getpagesize

Rawhide is getting cranky with posix compliance, and a few
things have stopped building.

getpagesize() is now only available -with- __USE_XOPEN_EXTENDED
or __USE_BSD, and NOT __USE_XOPEN2K.

_GNU_SOURCE must define __USE_XOPEN2K because getpagesize()
has gone away for mkfs.  I gave up and used sysconf.

Also, something used to pull in stat that no longer does, so
things like S_ISREG weren't getting defined.

The following fixes things for me.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>