xfs: fix up inode32/64 (re)mount handling

inode32/inode64 allocator behavior with respect to mount, remount
and growfs is a little tricky.

The inode32 mount option should only enable the inode32 allocator
heuristics if the filesystem is large enough for 64-bit inodes to
exist.  Today, it has this behavior on the initial mount, but a
remount with inode32 unconditionally changes the allocation
heuristics, even for a small fs.

Also, an inode32 mounted small filesystem should transition to the
inode32 allocator if the filesystem is subsequently grown to a
sufficient size.  Today that does not happen.

This patch consolidates xfs_set_inode32 and xfs_set_inode64 into a
single new function, and moves the "is the maximum inode number big
enough to matter" test into that function, so it doesn't rely on the
caller to get it right - which remount did not do, previously.

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

diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
index b570984..af01458 100644
--- a/fs/xfs/xfs_mount.h
+++ b/fs/xfs/xfs_mount.h
@@ -166,9 +166,8 @@
 #define XFS_MOUNT_GRPID		(1ULL << 9)	/* group-ID assigned from directory */
 #define XFS_MOUNT_NORECOVERY	(1ULL << 10)	/* no recovery - dirty fs */
 #define XFS_MOUNT_DFLT_IOSIZE	(1ULL << 12)	/* set default i/o size */
-#define XFS_MOUNT_32BITINODES	(1ULL << 14)	/* do not create inodes above
-						 * 32 bits in size */
-#define XFS_MOUNT_SMALL_INUMS	(1ULL << 15)	/* users wants 32bit inodes */
+#define XFS_MOUNT_SMALL_INUMS	(1ULL << 14)	/* user wants 32bit inodes */
+#define XFS_MOUNT_32BITINODES	(1ULL << 15)	/* inode32 allocator active */
 #define XFS_MOUNT_NOUUID	(1ULL << 16)	/* ignore uuid during mount */
 #define XFS_MOUNT_BARRIER	(1ULL << 17)
 #define XFS_MOUNT_IKEEP		(1ULL << 18)	/* keep empty inode clusters*/