Use dev_t for devnm2devid and devid2devnm

Use dev_t for devnm2devid and devid2devnm

Commit 4dd2df0966ec added a trip through makedev(), major(), and minor() for
device major and minor numbers. This would cause mdadm to fail in operating
on a device with a minor number bigger than (2^19)-1 due to it changing
from dev_t to a signed int and back.

Where this was found as a problem was when a array was created with a device
specified as a name like /dev/md/raidname and there were already 128 arrays
on the system. In this case, mdadm would chose 1048575 ((2^20)-1) for the
array and minor number. This would cause the major and minor number to become
negative when generated from devnm2devid() and passed to major() and minor()
in open_dev_excl(). open_dev_excl() would then call dev_open() which would
detect the negative minor number and call open() on the *char containing the
major:minor pair which isn't a valid file.

Signed-off-by: Mike Lovell <mlovell@bluehost.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>