xfs: split and refactor zone validation

xfs: split and refactor zone validation

Currently xfs_zone_validate mixes validating the software zone state in
the XFS realtime group with validating the hardware state reported in
struct blk_zone and deriving the write pointer from that.

Move all code that works on the realtime group to xfs_init_zone, and only
keep the hardware state validation in xfs_zone_validate.  This makes the
code more clear, and allows for better reuse in userspace.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>