resize2fs: avoid allocating over the MMP block

resize2fs: avoid allocating over the MMP block

When resizing past the point where the reserve inode has reserved
space for the block group descriptors to expand, and resize2fs (in an
offline resize) needs to move the allocation bitmaps and/or inode
table around, it's possible for resize2fs to allocate over the MMP
block, which would be bad.

Prevent this from happening by reserving the MMP block as a file
system metadata block (which it is) in resize2fs's accounting.

Addresses-Debian-Bug: #984472
Signed-off-by: Theodore Ts'o <tytso@mit.edu>