cls/cmpomap: add cls module for CMPXATTR-like functionality in omap

cls/cmpomap: add cls module for CMPXATTR-like functionality in omap

provides a cmp_vals() op similar to librados' omap_cmp(), but with full
support for the comparison modes and operations from cmpxattr(). if any
of the requested key/value comparisons are unsuccessful, the op fails
with -ECANCELED

cmp_vals() can be composed with other ops like set_omap_vals() to make
make them conditional on the successful comparison of all existing values

also provides cmp_set_vals() and cmp_rm_keys(), which apply a mutation
to only the keys that compare successfully. these enable batch
operations where, unlike cmp_vals() + set_omap_vals(), you don't want
one failed comparison to prevent operations on unrelated keys

Signed-off-by: Casey Bodley <cbodley@redhat.com>