xfs: clean up FS_XFLAG_REALTIME handling in xfs_ioctl_setattr_xflags

Introduce a local boolean variable if FS_XFLAG_REALTIME to make the
checks for it more obvious, and de-densify a few of the conditionals
using it to make them more readable while at it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20231025141020.192413-4-hch@lst.de


Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>