btrfs-progs: update btrfs_read_sys_array to match kernel implementation

btrfs-progs: update btrfs_read_sys_array to match kernel implementation

There are some sanity checks missing on both sides, kernel/userspace.
Preparation to port the missing changes.

Sync code with parent of kernel commit
1ffb22cf8c322bbfea6b35fe23d025841b49fede ("btrfs: cleanup, rename a few
variables in btrfs_read_sys_array")

This effectively reverts progs commit
be96777126d283773e4397278a5662d90676ab88 ("btrfs-progs: Cleanup unneeded
extra variant in btrfs_read_sys_array") so we can apply more of the
kernel patches.

Signed-off-by: David Sterba <dsterba@suse.com>