xfs: refactor non-power-of-two alignment checks

Create a helper function that can compute if a 64-bit number is an
integer multiple of a 32-bit number, where the 32-bit number is not
required to be an even power of two.  This is needed for some new code
for the realtime device, where we can set 37k allocation units and then
have to remap them.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/xfs_file.c

@@ -47,15 +47,9 @@ xfs_is_falloc_aligned(
 {
 	unsigned int		alloc_unit = xfs_inode_alloc_unitsize(ip);
 
-	if (!is_power_of_2(alloc_unit)) {
-		u32	mod;
-
-		div_u64_rem(pos, alloc_unit, &mod);
-		if (mod)
-			return false;
-		div_u64_rem(len, alloc_unit, &mod);
-		return mod == 0;
-	}
+	if (!is_power_of_2(alloc_unit))
+		return isaligned_64(pos, alloc_unit) &&
+		       isaligned_64(len, alloc_unit);
 
 	return !((pos | len) & (alloc_unit - 1));
 }

fs/xfs/xfs_linux.h

@@ -198,6 +198,11 @@ static inline uint64_t howmany_64(uint64_t x, uint32_t y)
 	return x;
 }
 
+static inline bool isaligned_64(uint64_t x, uint32_t y)
+{
+	return do_div(x, y) == 0;
+}
+
 /* If @b is a power of 2, return log2(b).  Else return -1. */
 static inline int8_t log2_if_power2(unsigned long b)
 {