mkfs: finobt mkfs support

mkfs: finobt mkfs support

Add the 'finobt' metadata option to mkfs to format an fs with free
inode btree support. If enabled, initialize the associated AGI
header fields and btree root block.

Also, do the initialization of the superblock version and feature
bits (including the new finobt flag) a bit earlier. These fields
must now be initialized prior to the use of XFS_PREALLOC_BLOCKS(),
as the latter returns a value that depends on whether a finobt root
btree block is reserved.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>