[GFS2] Remove iflags.h, use FS_

Update GFS2 in the light of David Howells' patch:

[PATCH] BLOCK: Move common FS-specific ioctls to linux/fs.h [try #6]
36695673

which calls the filesystem independant flags FS_..._FL. As a result
we no longer need the flags.h file and the conversion routine is
moved into the GFS2 source code.

Userland programs which used to include iflags.h should now include
fs.h and use the new flag names.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>