ext4: mballoc: add blocks to PA list under same spinlock after allocating blocks

ext4_mb_discard_preallocations() only checks for grp->bb_prealloc_list
of every group to discard the group's PA to free up the space if
allocation request fails. Consider below race:-

Process A  				Process B

1. allocate blocks
					1. Fails block allocation from
					     ext4_mb_regular_allocator()
   ext4_lock_group()
	allocated blocks
	more than ac_o_ex.fe_len
   ext4_unlock_group()
					2. Scans the
					   grp->bb_prealloc_list (under
					   ext4_lock_group()) and
					   find nothing and thus return
					   -ENOSPC.

2. Add the additional blocks to PA list

   ext4_lock_group()
   	add blocks to grp->bb_prealloc_list
   ext4_unlock_group()

Above race could be avoided if we add those additional blocks to
grp->bb_prealloc_list at the same time with block allocation when
ext4_lock_group() was still held.
With this discard-PA will know if there are actually any blocks which
could be freed from the PA

Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
Link: https://lore.kernel.org/r/a2217dd782585b42328981832e6d396abaaccb80.1589955723.git.riteshh@linux.ibm.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>