[XFS] break out the allocator specific parts of the xfs I/O path into

a separate file, xfs_iomap.c out of xfs_lrw.c. Remove some parts
of the code which were not doing anything for us. This is step
one in some major reorgs of this code.

SGI Modid: 2.5.x-xfs:slinx:131524a

fs/xfs/Makefile

@@ -127,6 +127,7 @@ xfs-objs			+= $(addprefix linux/, \
 				   xfs_fs_subr.o \
 				   xfs_globals.o \
 				   xfs_ioctl.o \
+				   xfs_iomap.o \
 				   xfs_iops.o \
 				   xfs_lrw.o \
 				   xfs_super.o \