rgw/common: rgw_bl_str(bl) avoids bl.c_str()

rgw/common: rgw_bl_str(bl) avoids bl.c_str()

where bl contains multiple buffer segments, c_str() has to
rellocate and copy those segments into a single buffer. use c_str()
instead, which just copies each segment into the resulting string

this allows the function to take the bufferlist argument by const ref

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