Tear down and flush the mmap region

The inode eviction should unmap the pages associated with the inode.
These pages should also be flushed to disk to avoid the data loss.
Therefore, use truncate_setsize() in evict_inode() to release the
pagecache.

The API truncate_setsize() was added in 2.6.35 kernel. To ensure
compatibility with the old kernel, the patch defines its own
truncate_setsize function.

Signed-off-by: Prasad Joshi <pjoshi@stec-inc.com>
Closes #255

config/kernel-truncate-setsize.m4

@@ -0,0 +1,12 @@
+dnl #
+dnl # 2.6.35 API change
+dnl # Added truncate_setsize() helper function.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_TRUNCATE_SETSIZE], [
+	ZFS_CHECK_SYMBOL_EXPORT(
+		[truncate_setsize],
+		[mm/truncate.c],
+		[AC_DEFINE(HAVE_TRUNCATE_SETSIZE, 1,
+		[truncate_setsize() is available])],
+		[])
+])

config/kernel.m4

@@ -37,6 +37,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_INSERT_INODE_LOCKED
 	ZFS_AC_KERNEL_D_OBTAIN_ALIAS
 	ZFS_AC_KERNEL_CHECK_DISK_SIZE_CHANGE
+	ZFS_AC_KERNEL_TRUNCATE_SETSIZE
 
 	if test "$LINUX_OBJ" != "$LINUX"; then
 		KERNELMAKE_PARAMS="$KERNELMAKE_PARAMS O=$LINUX_OBJ"