ext4: Avoid underflow in ext4_trim_fs()

Currently if len argument in ext4_trim_fs() is smaller than one block,
the 'end' variable underflow. Avoid that by returning EINVAL if len is
smaller than file system block.

Also remove useless unlikely().

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org