btrfs-progs: treat super.magic as an le64

btrfs-progs: treat super.magic as an le64

The super block magic is a le64 whose value looks like an unterminated
string in memory.  The lack of null termination leads to clumsy use of
string functions and causes static analysis tools to warn that the
string will be unterminated.

So let's just treat it as the le64 that it is.  Endian wrappers are used
on the constant so that they're compiled into run-time constants.

Signed-off-by: Zach Brown <zab@redhat.com>