ext4: fix reference counting bug on block allocation error

ext4: fix reference counting bug on block allocation error

commit 554a5ccc4e4a20c5f3ec859de0842db4b4b9c77e upstream.

If we hit this error when mounted with errors=continue or
errors=remount-ro:

    EXT4-fs error (device loop0): ext4_mb_mark_diskspace_used:2940: comm ext4.exe: Allocating blocks 5090-6081 which overlap fs metadata

then ext4_mb_new_blocks() will call ext4_mb_release_context() and try to
continue. However, ext4_mb_release_context() is the wrong thing to call
here since we are still actually using the allocation context.

Instead, just error out. We could retry the allocation, but there is a
possibility of getting stuck in an infinite loop instead, so this seems
safer.

[ Fixed up so we don't return EAGAIN to userspace. --tytso ]

Fixes: 8556e8f3b6 ("ext4: Don't allow new groups to be added during block allocation")
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
[bwh: Backported to 3.2:
 - Use EIO instead of EFSCORRUPTED
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>