xfs_repair: open filesystem device exclusively

xfs_repair: open filesystem device exclusively

Currently two xfs_repair processes will happily operate on the same
filesystem device at the same time.  It is also possible to mount a
filesystem that is in the process of being repaired.

This patch modifies xfs_repair to open the filesystem devices with
O_EXCL unless it was invoked in "no modify" or "dangerous" mode.

The net effect is that a 2nd xfs_repair will now safely fail with
"xfs_repair: cannot open /dev/foo: Device or resource busy", and a mount
command will fail with (the slightly cryptic) "mount: /dev/foo already
mounted or /mountpoint busy".

Note that this has no effect if the filesystem is stored in a regular
file instead of on a block device.

(Error messages could probably be improved to be more user-friendly in
this new failure case, and it probably wouldn't hurt to add a BLKROGET
ioctl to check for read-only block devices with read-write permissions,
but this does the job for me.)

Signed-off-by: Nathaniel W. Turner <nate@houseofnate.net>
Signed-off-by: Christoph Hellwig <hch@lst.de>