Btrfs: Fix sparse endian warnings in struct-funcs.c

Btrfs: Fix sparse endian warnings in struct-funcs.c

The btrfs macros to access individual struct members on disk were
sending the same variable to functions that expected different types
of endianness.  This fix explicitly creates a variable of the correct
type instead of abusing a single variable for mixed purposes.

Signed-off-by: Chris Mason <chris.mason@oracle.com>