ext4: handle errors in ext4_clear_blocks()

Checking return code from ext4_journal_get_write_access() is important
with snapshots, because this function invokes COW, so may return new
errors, such as ENOSPC.

ext4_clear_blocks() now returns < 0 for fatal errors, in which case,
ext4_free_data() is aborted.

Signed-off-by: Amir Goldstein <amir73il@users.sf.net>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>