mmc: core: Use PF_MEMALLOC flag for clock scaling context

Memory allocations with GFP_KERNEL flag in clock scaling path might
trigger IO transfer. This can cause deadlock since the claim_host
lock which is needed by mmc driver to perform IO is held by clock
scaling context.

Below is the exact call sequence:

mmc_devfreq_set_target() -> mmc_clk_update_freq () ->
mmc_change_bus_speed() -> mmc_select_hs400() -> mmc_select_bus_width()
-> mmc_get_ext_csd() -> kzalloc(512, GFP_KERNEL) -> try_to_free_pages()
-> evict() -> ext4_evict_inode().

To avoid this scenario, use PF_MEMALLOC flag for clock scaling context
so that it can have access to the more reserves of memory. And since
devfreq workitem would run in different kworker threads, restore to
original flags once done with setting clock frequency.

Change-Id: If52f83f6ecae982d8ed709b08fd84545c7ecbc43
Signed-off-by: Veerabhadrarao Badiganti <vbadigan@codeaurora.org>