debugfs: fix filehandle leak in copy_file()

debugfs: fix filehandle leak in copy_file()

Fix a file handle leak for the target file in copy_file() when error
handlers return without closing the file.  Instead, clean up at the
end of the function to handle cleanup in normal and error cases.

Minor other code style cleanups.

Signed-off-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>