ext4: Fix the NULL reference in double_down_write_data_sem()

ext4: Fix the NULL reference in double_down_write_data_sem()

commit 7247c0caa23d94a1cb6b307edba9dc45fb0798d4 upstream (as of v2.6.33-git11)

If EXT4_IOC_MOVE_EXT ioctl is called with NULL donor_fd, fget() in
ext4_ioctl() gets inappropriate file structure for donor; so we need
to do this check earlier, before calling double_down_write_data_sem().

Signed-off-by: Akira Fujita <a-fujita@rs.jp.nec.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>