iomap: move the swapfile code into a separate file

Move the swapfile activation code into a separate file so that we can
group related functions in a single file instead of having a single
enormous source file.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
diff --git a/fs/iomap/Makefile b/fs/iomap/Makefile
index de5a1f9..f88eca2 100644
--- a/fs/iomap/Makefile
+++ b/fs/iomap/Makefile
@@ -3,3 +3,6 @@
 # Copyright (c) 2019 Oracle.
 # All Rights Reserved.
 #
+obj-$(CONFIG_FS_IOMAP)		+= iomap.o
+
+iomap-$(CONFIG_SWAP)		+= swapfile.o