Add zfs_sb_prune_aliases() function

For kernels which do not implement a per-suberblock shrinker,
those older than Linux 3.1, the shrink_dcache_parent() function
was used to attempt to reclaim dentries.  This was found not be
entirely reliable and could lead to performance issues on older
kernels running meta-data heavy workloads.

To address this issue a zfs_sb_prune_aliases() function has been
added to implement this functionality.  It relies on traversing
the list of znodes for a filesystem and adding them to a private
list with a reference held.  The private list can then be safely
walked outside the z_znodes_lock to prune dentires and drop the
last reference so the inode can be freed.

This provides the same synchronous behavior as the per-filesystem
shrinker and has the advantage of depending on only long standing
interfaces.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Closes #3501

config/kernel-d-prune-aliases.m4

@@ -0,0 +1,19 @@
+dnl #
+dnl # 2.6.12 API change
+dnl # d_prune_aliases() helper function available.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_D_PRUNE_ALIASES],
+	[AC_MSG_CHECKING([whether d_prune_aliases() is available])
+	ZFS_LINUX_TRY_COMPILE_SYMBOL([
+		#include <linux/dcache.h>
+	], [
+		struct inode *ip = NULL;
+		d_prune_aliases(ip);
+	], [d_prune_aliases], [fs/dcache.c], [
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_D_PRUNE_ALIASES, 1,
+		          [d_prune_aliases() is available])
+	], [
+		AC_MSG_RESULT(no)
+	])
+])

config/kernel.m4

@@ -78,6 +78,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_INSERT_INODE_LOCKED
 	ZFS_AC_KERNEL_D_MAKE_ROOT
 	ZFS_AC_KERNEL_D_OBTAIN_ALIAS
+	ZFS_AC_KERNEL_D_PRUNE_ALIASES
 	ZFS_AC_KERNEL_D_SET_D_OP
 	ZFS_AC_KERNEL_D_REVALIDATE_NAMEIDATA
 	ZFS_AC_KERNEL_CONST_DENTRY_OPERATIONS