dm-verity-fec: use standard names for Reed-Solomon parameters

dm-verity-fec: use standard names for Reed-Solomon parameters

"RS(n, k)" is by far the most common and standard notation for
describing Reed-Solomon codes.  Each RS codeword consists of 'n'
symbols, divided into 'k' message symbols and 'n - k' parity symbols.
'n - k' is also the number of roots of the generator polynomial.

dm-verity uses "RS(M, N)" instead.  I haven't been able to find any
other source that uses this convention.  This quirk makes the code
harder to understand than necessary, especially due to dm-verity's 'N'
meaning something different from the standard 'n'.

Therefore, update dm-verity-fec.c and dm-verity-fec.h to use the
standard parameter names.  No functional changes.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>