Add SEEK_DATA/SEEK_HOLE to lseek()/llseek()

The approach taken was the rework zfs_holey() as little as
possible and then just wrap the code as needed to ensure
correct locking and error handling.

Tested with xfstests 285 and 286.  All tests pass except for
7-9 of 285 which try to reserve blocks first via fallocate(2)
and fail because fallocate(2) is not yet supported.

Note that the filp->f_lock spinlock did not exist prior to
Linux 2.6.30, but we avoid the need for autotools check by
virtue of the fact that SEEK_DATA/SEEK_HOLE support was not
added until Linux 3.1.

An autoconf check was added for lseek_execute() which is
currently a private function but the expectation is that it
will be exported perhaps as early as Linux 3.11.

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1384

config/kernel-lseek-execute.m4

@@ -0,0 +1,23 @@
+dnl #
+dnl # 3.11 API change
+dnl # lseek_execute helper exported
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_LSEEK_EXECUTE],
+	[AC_MSG_CHECKING([whether lseek_execute() is available])
+	ZFS_LINUX_TRY_COMPILE_SYMBOL([
+		#include <linux/fs.h>
+	], [
+		struct file *fp __attribute__ ((unused)) = NULL;
+		struct inode *ip __attribute__ ((unused)) = NULL;
+		loff_t offset __attribute__ ((unused)) = 0;
+		loff_t maxsize __attribute__ ((unused)) = 0;
+
+		lseek_execute(fp, ip, offset, maxsize);
+	], [lseek_exclusive], [fs/read_write.c], [
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_LSEEK_EXECUTE, 1,
+		          [lseek_execute() is available])
+	], [
+		AC_MSG_RESULT(no)
+	])
+])

config/kernel.m4

@@ -77,6 +77,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_SET_NLINK
 	ZFS_AC_KERNEL_ELEVATOR_CHANGE
 	ZFS_AC_KERNEL_5ARG_SGET
+	ZFS_AC_KERNEL_LSEEK_EXECUTE
 
 	AS_IF([test "$LINUX_OBJ" != "$LINUX"], [
 		KERNELMAKE_PARAMS="$KERNELMAKE_PARAMS O=$LINUX_OBJ"