mkfs: substitute slashes with spaces in protofiles

mkfs: substitute slashes with spaces in protofiles

A user requested the ability to specify directory entry names in a
protofile that have spaces in them.  The protofile format itself does
not allow spaces (yay 1973-era protofiles!) but it does allow slashes.
Slashes aren't allowed in directory entry names, so we'll permit this
one gross hack.  After this, the protofile:

/
0 0
d--775 1000 1000
: Descending path /code/t/fstests
 get/isk.sh   ---775 1000 1000 /code/t/fstests/getdisk.sh
$

Will produce "get isk.h" in the root directory when used thusly:

# mkfs.xfs -p slashes_are_spaces=1,/tmp/protofile -f /dev/sda

Requested-by: Daan De Meyer <daan.j.demeyer@gmail.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>