ext4: don't allow ext4_free_blocks() to fail due to ENOMEM

commit e7676a704ee0a1ef71a6b23760b5a8f6896cb1a1 upstream.

The filesystem should not be marked inconsistent if ext4_free_blocks()
is not able to allocate memory.  Unfortunately some callers (most
notably ext4_truncate) don't have a way to reflect an error back up to
the VFS.  And even if we did, most userspace applications won't deal
with most system calls returning ENOMEM anyway.

Reported-by: Nagachandra P <nagachandra@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>