crimson/osd/ec_backend: fix OP_OMAP_RMKEYRANGE translation

crimson/osd/ec_backend: fix OP_OMAP_RMKEYRANGE translation

transate_transaction() handled OP_OMAP_RMKEYRANGE in the same case
block as OP_OMAP_SETHEADER, decoding a single bufferlist and calling
omap_setheader(). But OP_OMAP_RMKEYRANGE encodes first/last as two
strings, so decode_bl() misreads the stream and the wrong op is
applied.

Give OP_OMAP_RMKEYRANGE its own case: decode the two strings and pass
them to the two-string PGTransaction::omap_rmkeyrange() overload, which
encodes them into the string[2] bufferlist internally.

Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>