f2fs: use inode_lock_shared instead of inode_lock in f2fs_seek_block()

inode_lock_shared() -> down_read(&inode->i_rwsem)
       inode_lock() -> down_write(&inode->i_rwsem)

Inode is not updated in f2fs_seek_block(), so there is no need
to hold write lock, use read lock for more efficiency.
Signed-off-by: zhangxirui <xirui.zhang@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -418,7 +418,7 @@ static loff_t f2fs_seek_block(struct file *file, loff_t offset, int whence)
 	loff_t isize;
 	int err = 0;
 
-	inode_lock(inode);
+	inode_lock_shared(inode);
 
 	isize = i_size_read(inode);
 	if (offset >= isize)
@@ -483,10 +483,10 @@ static loff_t f2fs_seek_block(struct file *file, loff_t offset, int whence)
 found:
 	if (whence == SEEK_HOLE && data_ofs > isize)
 		data_ofs = isize;
-	inode_unlock(inode);
+	inode_unlock_shared(inode);
 	return vfs_setpos(file, data_ofs, maxbytes);
 fail:
-	inode_unlock(inode);
+	inode_unlock_shared(inode);
 	return -ENXIO;
 }