Btrfs: Force stripesize to the value of sectorsize

Btrfs code currently assumes stripesize to be same as
sectorsize. However Btrfs-progs (until commit
df05c7ed455f519e6e15e46196392e4757257305) has been setting
btrfs_super_block->stripesize to a value of 4096.

This commit makes sure that the value of btrfs_super_block->stripesize
is a power of 2. Later, it unconditionally sets btrfs_root->stripesize
to sectorsize.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>