cluster/distribute: Rebalance should also verify free inodes

Currently during `MIGRATE_DATA` we never verified about the total
inode usage among new and old bricks. Such checks are available for
disk space usage but it is also needed for inodes during data
migration. Such a check leads to uniform outcome of file distribution
upon rebalance.

Patch provides:

- Check dst_inodes < src_inodes, a friendly `warning` message to
  indicate we have to ignore such an attempt
- Rename __dht_check_free_space() --> __dht_check_free_space_and_inodes()

Change-Id: I7bc4dd8b507883f0fb836300e99f0bb083493f5f
BUG: 982919
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/5948
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>
Revert "cluster/distribute: Rebalance should also verify free inodes"

This reverts commit 215fea41a96479312a5ab8783c13b30ab9fe00fa

Realized soon after merging, that this patch is actually useless.
Checking for inode utilization on dst relative to src is of no use
because dst is already storing linkfile and inode is already consumed
no matter what. We might as well perform the rebalance and save on an
inode on the src node.

Change-Id: I7b8b8d35d9063a710e1bee1c7c6c7739fcc45ad4
Reviewed-on: http://review.gluster.org/5960
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Tested-by: Anand Avati <avati@redhat.com>
build: AM_PATH_XML2 needs proactive check

AM_PATH_XML2 needs check for its existence using
aclocal macros - to avoid problems like below when
libxml2 development libraries are not installed

--------------------------------------------------

... GlusterFS autogen ...

Generate gf-error-codes.h ...
`gf-error-codes.h' -> `libglusterfs/src/gf-error-codes.h'
Running aclocal...
configure.ac:524: warning: macro `AM_PATH_XML2' not found in library
Running autoheader...
Running libtoolize...
Running autoconf...
configure.ac:524: error: possibly undefined macro: AM_PATH_XML2
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.
Running automake...
--------------------------------------------------

Change-Id: Ife463c34c45babc1c4c0ed6e8128b5c43419b9b9
BUG: 947226
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/5962
Reviewed-by: Kaushal M <kaushal@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>