ext4: add a function merging extents right and left
ext4: add a function merging extents right and left

1) Rename ext4_ext_try_to_merge() to ext4_ext_try_to_merge_right().

2) Add a new function ext4_ext_try_to_merge() which tries to merge
   an extent both left and right.

3) Use the new function in ext4_ext_convert_unwritten_endio() and
   ext4_ext_insert_extent().

Signed-off-by: Yongqiang Yang <cc47c8d45a0a3c6300393e2d0c4f6e661d0cfdb0@gmail.com>
Tested-by: Allison Henderson <f908fb55b00b5234257aee48327eaf34b75167a0@linux.vnet.ibm.com>

KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs