libext2fs, libe2p: Reserve RO_COMPAT_REPLICA feature

libext2fs, libe2p: Reserve RO_COMPAT_REPLICA feature

The replica is a feature which stores multiple copies of the key
metadata blocks so a single block failure in failure-prone media
(read: certain types of flash storage) doesn't take out the entire
file system.

Discussion on the upstream list proved not to be very positive on this
feature; the arguments were that it added complexity that wasn't
warrented, since common practice in industry is to insist on reliable
media, and if media is unreliable, you're kind of toast anyway (unless
the file system is being used as the back-end store of a cluster file
system where checksuming and data replication is happening above the
local disk file system level).  So, this feature is being developed
out of tree.

We reserve the code points so that other people won't accidentally
step on them.  Since it's not upstream, it's a soft reservation, but
it's not like we have any shortage of RO_COMPAT features.  We are a
bit more tight on reserved inodes, but EXT2_BOOT_LOADER_INO and
EXT2_UNDEL_DIR_INO are not currently used anywhere, and
EXT2_EXCLUDE_INO is a reservation for another out-of-tree feature.
There are no features currently being discussed which require a
reserved inode, but if a need were to arise, we can claw back code
point reservations that were never used or not in tree, as those will
always be considered lower priority than in-tree features.

Cc: Aditya Kali <adityakali@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>