Fix statfs(2) for 32-bit user space

When handling a 32-bit statfs() system call the returned fields,
although 64-bit in the kernel, must be limited to 32-bits or an
EOVERFLOW error will be returned.

This is less of an issue for block counts since the default
reported block size in 128KiB. But since it is possible to
set a smaller block size, these values will be scaled as
needed to fit in a 32-bit unsigned long.

Unlike most other filesystems the total possible file counts
are more likely to overflow because they are calculated based
on the available free space in the pool. In order to prevent
this the reported value must be capped at 2^32-1. This is
only for statfs(2) reporting, there are no changes to the
internal ZFS limits.

Reviewed-by: Andreas Dilger <andreas.dilger@whamcloud.com>
Reviewed-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #7927 
Closes #7122 
Closes #7937

module/zfs/zpl_super.c

@@ -181,6 +181,28 @@ zpl_statfs(struct dentry *dentry, struct kstatfs *statp)
 	spl_fstrans_unmark(cookie);
 	ASSERT3S(error, <=, 0);
 
+	/*
+	 * If required by a 32-bit system call, dynamically scale the
+	 * block size up to 16MiB and decrease the block counts.  This
+	 * allows for a maximum size of 64EiB to be reported.  The file
+	 * counts must be artificially capped at 2^32-1.
+	 */
+	if (unlikely(zpl_is_32bit_api())) {
+		while (statp->f_blocks > UINT32_MAX &&
+		    statp->f_bsize < SPA_MAXBLOCKSIZE) {
+			statp->f_frsize <<= 1;
+			statp->f_bsize <<= 1;
+
+			statp->f_blocks >>= 1;
+			statp->f_bfree >>= 1;
+			statp->f_bavail >>= 1;
+		}
+
+		uint64_t usedobjs = statp->f_files - statp->f_ffree;
+		statp->f_ffree = MIN(statp->f_ffree, UINT32_MAX - usedobjs);
+		statp->f_files = statp->f_ffree + usedobjs;
+	}
+
 	return (error);
 }