ext4: fix check of dqget() return value in ext4_ioctl_setproject()

commit ff0bc084 upstream.

A failed call to dqget() returns an ERR_PTR() and not null. Fix
the check in ext4_ioctl_setproject() to handle this correctly.

Fixes: 9b7365fc ("ext4: add FS_IOC_FSSETXATTR/FS_IOC_FSGETXATTR interface support")
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>