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 29a814d2ee0e43c2980f33f91c1311ec06c0aa35 (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
a1d6cc563bfdf1bf2829d3e6ce4d8b774251796b (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>