xfs_io: allow open file permissions to be changed

xfs_io: allow open file permissions to be changed

I need to be able to open a file read-write, then change the
permissions on the file to read-only to check that copy_file_range
returns EPERM correctly in that case. This can't be done as root,
because root ignores file permissions, but as a normal user we can't
open a 0444 file for writing and so can't actually test writing to
a read-only file without some method of "open read-write, change
permissions to read-only, try to write to file through open
read-write file".

So, allow adding or removing write permissions on an open file.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
[sandeen: Move man page entry to FILE section]
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>