ext4: Convert truncate_mutex to read write semaphore.

We are currently taking the truncate_mutex for every read. This would have
performance impact on large CPU configuration. Convert the lock to read write
semaphore and take read lock when we are trying to read the file.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

include/linux/ext4_fs_i.h

@@ -139,16 +139,16 @@ struct ext4_inode_info {
 	__u16 i_extra_isize;
 
 	/*
-	 * truncate_mutex is for serialising ext4_truncate() against
+	 * i_data_sem is for serialising ext4_truncate() against
 	 * ext4_getblock().  In the 2.4 ext2 design, great chunks of inode's
 	 * data tree are chopped off during truncate. We can't do that in
 	 * ext4 because whenever we perform intermediate commits during
 	 * truncate, the inode and all the metadata blocks *must* be in a
 	 * consistent state which allows truncation of the orphans to restart
 	 * during recovery.  Hence we must fix the get_block-vs-truncate race
-	 * by other means, so we have truncate_mutex.
+	 * by other means, so we have i_data_sem.
 	 */
-	struct mutex truncate_mutex;
+	struct rw_semaphore i_data_sem;
 	struct inode vfs_inode;
 
 	unsigned long i_ext_generation;