disk: efi: Fix memory leak on 'gpt verify'

disk: efi: Fix memory leak on 'gpt verify'

Below is what happens on R-Car H3ULCB-KF using clean U-Boot
v2019.04-00810-g6aebc0d11a10 and r8a7795_ulcb_defconfig:

 => ### interrupt autoboot
 => gpt verify mmc 1
 No partition list provided - only basic check
 Verify GPT: success!
 => ### keep calling 'gpt verify mmc 1'
 => ### on 58th call, we are out of memory:
 => gpt verify mmc 1
 alloc_read_gpt_entries: ERROR: Can't allocate 0X4000 bytes for GPT Entries
 GPT: Failed to allocate memory for PTE
 gpt_verify_headers: *** ERROR: Invalid Backup GPT ***
 Verify GPT: error!

This is caused by calling is_gpt_valid() twice (hence allocating pte
also twice via alloc_read_gpt_entries()) while freeing pte only _once_
in the caller of gpt_verify_headers(). Fix that by freeing the pte
allocated and populated for primary GPT _before_ allocating and
populating the pte for backup GPT. The latter will be freed by the
caller of gpt_verify_headers().

With the fix applied, the reproduction scenario [1-2] has been run
hundreds of times in a loop w/o running into OOM.

[1] gpt verify mmc 1
[2] gpt verify mmc 1 $partitions

Fixes: cef68bf9042dda ("gpt: part: Definition and declaration of GPT verification functions")
Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>