kernel/linux: fix using custom location

Currently, extract and patch are skipped as thus:
- using a custom directory of pre-installed headers
- a correctly named directory already exists

Otherwise, extract and patch are done.

The current second condition is wrong, because it allows the following
sequence to happen:
- a non-custom kernel is used
- a previous build only partially extracted the non-custom sources
- that p[revious build broke during extraction (eg. incomplete tarball...)
- a subsequent build will find a properly named directory, and will
thus skip extract and patch, which is wrong

Fix that by following the conditions in this table:

Type | Extract | Patch
----------------------+---------+-------
Pre-installed headers | N | N
custom directory | N | N
custom tarball | Y | N
mainstream tarball | Y | Y

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: David Holsgrove <david.holsgrove@xilinx.com>

 diff --git a/mpn/generic/toom44_mul.c b/mpn/generic/toom44_mul.c

 --- a/mpn/generic/toom44_mul.c

 +++ b/mpn/generic/toom44_mul.c

 @@ -116,17 +116,16 @@

    TMP_MARK;

 -  as1  = TMP_SALLOC_LIMBS (n + 1);

 -  asm1 = TMP_SALLOC_LIMBS (n + 1);

 -  as2  = TMP_SALLOC_LIMBS (n + 1);

 -  ash  = TMP_SALLOC_LIMBS (n + 1);

 -  asmh = TMP_SALLOC_LIMBS (n + 1);

 -

 -  bs1  = TMP_SALLOC_LIMBS (n + 1);

 -  bsm1 = TMP_SALLOC_LIMBS (n + 1);

 -  bs2  = TMP_SALLOC_LIMBS (n + 1);

 -  bsh  = TMP_SALLOC_LIMBS (n + 1);

 -  bsmh = TMP_SALLOC_LIMBS (n + 1);

 +  as1  = TMP_ALLOC_LIMBS (10 * n + 10);

 +  asm1 = as1  + n + 1;

 +  as2  = asm1 + n + 1;

 +  ash  = as2  + n + 1;

 +  asmh = ash  + n + 1;

 +  bs1  = asmh + n + 1;

 +  bsm1 = bs1  + n + 1;

 +  bs2  = bsm1 + n + 1;

 +  bsh  = bs2  + n + 1;

 +  bsmh = bsh  + n + 1;

    gp = pp;

    hp = pp + n + 1;