UBIFS: fix GC LEB recovery

UBIFS: fix GC LEB recovery

UBIFS tries to alway have an LEB reserved for GC, and stores it
in c->gc_lnum. Besides, there is GC head which points to the current
GC head LEB.

In case of an unclean power cut, what may happen is that the GC head
was switched to the reserved GC LEB (c->gc_lnum), but a new reserved
GC LEB was not created yet. So, after an unclean reboot we may have
no reserved GC LEB, and we need to find a new LEB for this.

To do this, we find a dirty LEB which can fit the current GC head,
move the data, unmap this dirty LEB, and it becomes our reserved GC
LEB.

However, if we cannot find a dirty enough LEB, we return failure,
which is wrong, because we still can have free LEBs to use for
the reserved GC LEB. This patch fixes the issue.

This patch also fixes few typos in comments, which were spotted by
aspell.

Note, this patch fixes a real issue

[   14.328117] UBIFS: recovery needed
[   53.941378] UBIFS error (pid 462): ubifs_rcvry_gc_commit: could not find a dirty LEB
[   89.606399] UBIFS: recovery completed
[   89.609329] UBIFS assert failed in mount_ubifs at 1358 (pid 462)
[   89.616165] [<c0026144>] (unwind_backtrace+0x0/0xe4) from [<c0125ce4>] (ubifs_fill_super+0x11d0/0x1c4c)
[   89.625930] [<c0125ce4>] (ubifs_fill_super+0x11d0/0x1c4c) from [<c0126910>] (ubifs_get_sb+0x1b0/0x354)
[   89.635696] [<c0126910>] (ubifs_get_sb+0x1b0/0x354) from [<c008a50c>] (vfs_kern_mount+0x50/0xe0)
[   89.644485] [<c008a50c>] (vfs_kern_mount+0x50/0xe0) from [<c008a5e0>] (do_kern_mount+0x34/0xdc)
[   89.653274] [<c008a5e0>] (do_kern_mount+0x34/0xdc) from [<c00a29d8>] (do_mount+0x148/0x7cc)
[   89.662063] [<c00a29d8>] (do_mount+0x148/0x7cc) from [<c00a30f4>] (sys_mount+0x98/0xc8)
[   89.670852] [<c00a30f4>] (sys_mount+0x98/0xc8) from [<c0021f40>] (ret_fast_syscall+0x0/0x28)

which was reported here:
http://article.gmane.org/gmane.linux.drivers.mtd/29923
by Alexander Pazdnikov <pazdnikov@list.ru>

Reported-by: Alexander Pazdnikov <pazdnikov@list.ru>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Reviewed-by: Adrian Hunter <adrian.hunter@nokia.com>