ext4: fix end of region partial cluster handling

ext4_ext_remove_space() can incorrectly free a partial_cluster if
EAGAIN is encountered while truncating or punching.  Extent removal
should be retried in this case.

It also fails to free a partial cluster when the punched region begins
at the start of a file on that unaligned cluster and where the entire
file has not been punched.  Remove the requirement that all blocks in
the file must have been freed in order to free the partial cluster.

Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>