Remove two unneeded exports and make two symbols static in fs/mpage.c

Remove two unneeded exports and make two symbols static in fs/mpage.c

Commit 3329c6461a6b20f85b258b48ee6b2f45a1c7ed3c (vfs: add hooks for
ext4's delayed allocation support) exported the following functions

mpage_bio_submit()
__mpage_writepage()

for the benefit of ext4's delayed allocation support. Since commit
3bce258f2bead2524cbe9ef9deff3d331bc2d9b7 (ext4: Rework the
ext4_da_writepages() function), these functions are not used by the
ext4 driver anymore. However, the now unnecessary exports still
remain, and this patch removes those. Moreover, these two functions
can become static again.

The issue was spotted by namespacecheck.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@movial.com>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>