xfsprogs: libxcmd: use fs_device_number() consistently

xfsprogs: libxcmd: use fs_device_number() consistently

The libxcmd code builds up a table that records information about
all filesystems that might be subject to quotas, as well as a set
of directories that are the roots of project quota trees.

When building the table, the device number for each affected
filesystem is determined (in fs_device_number()) using a call to
stat64().  It turns out that in all cases when doing this, a
directory path (and *not* a device special file path) is specified,
in which case the appropriate filesystem device id is found in the
st_dev field produce by the call to stat64() (i.e., the device id
for the mounted filesystem containing the path).  Accordingly,
fs_device_number() always returns the st_dev field.

Another routine, fs_table_lookup(), looks up an entry in this table
based on the path name provided.  However this function allows a
path to a device special file be provided.  In that case the right
device id to use is found in the st_rdev field returned by stat64().

I found this to be confusing, and it took a while to convince
myself that this wasn't actually bug.  (It wasn't initially clear
that device special files were never passed to fs_device_number().)

In order to prevent myself and others from ever wasting time like
this again, use fs_device_number() every time a device number is
needed, and in doing so determine it consistently in all cases (that
is--use st_rdev for device special files and st_dev otherwise).

In the process, change fs_device_number() to return an zero on
success (or an errno) rather than its first argument (or NULL).

Signed-off-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>