device: categorise block i/o

device: categorise block i/o

Introduce enum dev_io_reason to categorise block device I/O
in debug messages so it's obvious what it is for.

DEV_IO_SIGNATURES   /* Scanning device signatures */
DEV_IO_LABEL        /* LVM PV disk label */
DEV_IO_MDA_HEADER   /* Text format metadata area header */
DEV_IO_MDA_CONTENT  /* Text format metadata area content */
DEV_IO_FMT1         /* Original LVM1 metadata format */
DEV_IO_POOL         /* Pool metadata format */
DEV_IO_LV           /* Content written to an LV */
DEV_IO_LOG          /* Logging messages */